Beginner Multiplying aperture by crop factor? True or not?

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Conan
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#1
Hi everyone, I have got my head around why for example a 50MM lens will appear around 75 or so MM on a APSC body due to the difference in sensor size vs 35MM full frame, but came across a video on youtube that says you should also multiply the aperture too.

I realise that a 50MM lens is a 50MM lens, or a 1.8 aperture is still a 1.8 aperture no matter what body it is used on, but wondered if it does indeed effect things as much as stated in the video.

It would be great if you could clear this up for me.

This is the video in case anyone wondered
View: https://www.youtube.com/watch?v=f5zN6NVx-hY


Thank you all for your help, I've learnt a lot from you all here so far.
 
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Robert
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#2
Just take photos and don't worry about it.
Unless you've spent years on full frame and are moving to m43, or use different systems with same lenses, it is all a bit irrelevant.
M43 dof is greater than ff dof for a given aperture.
So they're suggesting that to get the effect of f5.6 on ff you should use f2.8 on m43.
 
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Alan
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#3
I'll try... You can apply the crop factor to the aperture when looking at depth of field but if you're just looking at exposure then you maybe shouldn't bother as the exposure at f1.8 or any other setting will be the same. But, if you're looking at image quality you can apply the crop factor again because of the effects having a smaller sensor probably with smaller pixels and with differing ISO performances could have on final image quality.

If you're using a 50mm f1.8 on FF you'll need a 33mm f1.2 on APS-C because 50/1.8 = 33.3/f1.2, assuming a crop factor of about 1.5. This will give you about the same field of view and depth of field because for any f number aperture in mm = focal length/fx so 50/1.8=27.7mm and 33/1.2=27.5mm which is near enough the same. You probably can't buy a 33mm f1.2 but you might be able to get a 35mm f1.2 so a 50mm f1.8 on FF will be about the same as a 35mm f1.2 on APS-C.

To make things a bit easier :D lets look at MFT with a x2 crop as the numbers are easy. To get about the same as a 50mm f1.8 on MFT you'll need a 25mm 0.9 and you can actually buy a MFT 25mm f0.95. I had one and it was a really nice lens :D I tend to apply the crop factor when using MFT and if for example I'd use a 50mm on FF at f4-8 I'll use a 25mm on MFT at f2-4.

I hope all that makes sense and that it helps, and I also hope it's actually about right :D
 
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Alan
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#5
As above shoot with what you have not what you could have. I use MFT and don't worry about comparing to full frame equivalent when out shooting.
I do know what you're saying but I think it's worth applying the crop factor as if for example, like me, you grew up with "FF" film and now use a digital FF and also swap to a smaller system you could wonder why the results are different. If I used MFT exactly as I do FF and shot at f4 to f8 most of the time the results would look quite different whereas if I shoot MFT at f2 to f4 the results can be very largely the same as I'd get with FF at f4 to f8.

And a PS to that.
Even if starting with and only experiencing a smaller system I think it's worth applying the crop factor to get the best results you can. For example I'm pretty sure that the image quality you'll get with a smaller system at f4 will be better than what you'd get at f8 as with the smaller systems you're probably running into diffraction issues at f8 and possibly raising the ISO too and both these things could impact on image quality. I think therefore it's not only about comparing smaller systems to larger ones it's also about understanding your kit and getting the best results out of the kit that you have regardless of what you could maybe get from a larger system.
 
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conanthewarrior
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Conan
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#7
Thank you all for your answers.

I am definitely shooting with what I have, I just like to understand things. I won't have a FF camera for a long time, so will enjoy what I have now :)
 
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#9
One time a focal length change will change the f no is on zoom lenses.

Which is why you will often see 2 2 f Nos quoted on the lens specs eg:

Canon EF 70-300mm f/4-5.6 IS II

The reason is that the longer end the effective aperture is f 5.6 as opposed to f 4.5 because the size of the widest aperture limits the aperture at all zoom lengths and thus limits the light available at the sensor.

Similarly using extension tubes will also decrease the light available as the length of the tubes increase.
 
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#10
Always ignore crop factors and get to know how the kit you actually have works. I shoot full frame, APS-C and MTF and never have a need to multiply anything by anything. The lens on my MTF works as it does, I do not need to know how it would work on my FF camera. My APS-C lenses work as they do and, again, I have no need to know how they would work on either the MTF or FF cameras.
 
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#11
Always ignore crop factors and get to know how the kit you actually have works. I shoot full frame, APS-C and MTF and never have a need to multiply anything by anything. The lens on my MTF works as it does, I do not need to know how it would work on my FF camera. My APS-C lenses work as they do and, again, I have no need to know how they would work on either the MTF or FF cameras.
But you might like to know how to get the best out of your kit. You can do that by trial and error and experience and by learning from changing settings and viewing the results or you can look at the theory and try to understand what's going on and then see the results in practice.

You can ignore how your system relates to another system but IMVHO the whole crop factor and comparison to FF thing does relate to image quality and even if you never intend to touch a larger system knowing what's going on and understanding why using certain settings will get you better quality results well help.

I've seen plenty of people post pictures taken with MTF at f11-16 when f4-5.6 would have been sufficient and would have given better final image quality. This might well show that some people could benefit from knowing how their kit could give them image quality closer to that which a larger system could get them. I think it's worth knowing.
 
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Ian
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#12
I do know what you're saying but I think it's worth applying the crop factor as if for example, like me, you grew up with "FF" film and now use a digital FF and also swap to a smaller system you could wonder why the results are different. If I used MFT exactly as I do FF and shot at f4 to f8 most of the time the results would look quite different whereas if I shoot MFT at f2 to f4 the results can be very largely the same as I'd get with FF at f4 to f8.

And a PS to that.
Even if starting with and only experiencing a smaller system I think it's worth applying the crop factor to get the best results you can. For example I'm pretty sure that the image quality you'll get with a smaller system at f4 will be better than what you'd get at f8 as with the smaller systems you're probably running into diffraction issues at f8 and possibly raising the ISO too and both these things could impact on image quality. I think therefore it's not only about comparing smaller systems to larger ones it's also about understanding your kit and getting the best results out of the kit that you have regardless of what you could maybe get from a larger system.
Very true the smaller sensor systems need to use the shallower depth of field to avoid defraction. It's a shame the kit lenses are still the usual 3.5-5.6 when most of the time something faster than 5.6 would be welcome.
 
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Robert
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#13
But you might like to know how to get the best out of your kit. You can do that by trial and error and experience and by learning from changing settings and viewing the results or you can look at the theory and try to understand what's going on and then see the results in practice.

You can ignore how your system relates to another system but IMVHO the whole crop factor and comparison to FF thing does relate to image quality and even if you never intend to touch a larger system knowing what's going on and understanding why using certain settings will get you better quality results well help.

I've seen plenty of people post pictures taken with MTF at f11-16 when f4-5.6 would have been sufficient and would have given better final image quality. This might well show that some people could benefit from knowing how their kit could give them image quality closer to that which a larger system could get them. I think it's worth knowing.
You can do all that without worrying about crop factors etc. You just need to know how to get the best out of your kit.
 

AndrewFlannigan

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#14
You just need to know how to get the best out of your kit.
Yes.

To put this in perspective: I've used 5x4, 6x9, 6x7, 6x6, 645, 35mm, FF, APS, M43 and even Minox sub miniature. To get the best out of your kit you MUST practice with each camera and lens. With digital it's easy: get out and take lots of pictures.
 
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Phil
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#16
Thank you all for your answers.

I am definitely shooting with what I have, I just like to understand things. I won't have a FF camera for a long time, so will enjoy what I have now :)
The understanding of 'things' doesn't really work when what's being described ends up as an abstract.

If you had a FF camera, it'd be handy to know that f/4 on the crop had a similar DoF to f/5.6 on the FF.

However, if you only have the crop - the knowledge that the DoF should take the crop factor into consideration is a bit pointless. Like me describing a £15 bottle of plonk as being 'not quite as appley as Crystal' if you'd not tasted Crystal. In itself describing the DoF in absolute mathematical terms is useful to some - but describing it purely in relation to something else you're not familiar with is pointless.
 
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conanthewarrior
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Conan
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#17
The understanding of 'things' doesn't really work when what's being described ends up as an abstract.

If you had a FF camera, it'd be handy to know that f/4 on the crop had a similar DoF to f/5.6 on the FF.

However, if you only have the crop - the knowledge that the DoF should take the crop factor into consideration is a bit pointless. Like me describing a £15 bottle of plonk as being 'not quite as appley as Crystal' if you'd not tasted Crystal. In itself describing the DoF in absolute mathematical terms is useful to some - but describing it purely in relation to something else you're not familiar with is pointless.
But the whole universe is pretty abstract, why not know?

I'm not familiar with god, but some people are, even if it seems unexplainable to me. This is something I can understand and does exist, something I was unsure of when I made this thread.

I hope that doesn't sound weird, it was the best comparison I could think of, not meant in a bad way.

EDIT: actually a better one would probably be imperial and metric. I am familiar with one, but know the other exists, so no harm knowing the other one.
 
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conanthewarrior
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Conan
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#18
I can't remember, what do you have?
I just have the 18-55MM kit lens with my D3500. I am buying a 50MM 1.8 next weekend, not sure what brand to go with yet. Might buy a few more lenses that are cheap but they can only be used MF on my body, but for the price might as well.
 
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Andrew
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#19
Three things affect depth of field, focal length of the lens, distance to subject and aperture.

One of the reasons that some people think you need to take in to account the sensor size is because they are trying to recreate the same photo, for example, you shoot a portrait with a 50mm on a full frame then switch to a crop sensor with a 50mm. In order to get the same composition you will need to move back which means you will need to change the aperture to maintain the depth of field.

But the need to change the aperture is because you moved, not because you changed the sensor
 
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Mark
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#20
For everyone saying don't worry about focal length (or more strictly FoV) and DoF equivalents, consider this...


You are at a fixed point and the subject is likewise at a fixed point. Lets take The Kia Oval, the press positions and the wickets as an easy example.

I'm using a Canon 500/4 and have two available bodies for that lens (as well as using one short and one wide lens on other bodies).

One is a 1Dx (FF) the other is a 7DII (APS-C - x1.6).

therefore I can either shoot at an FoV equivalent to 500mm @ f/4 or 800mm at f/5.6.

In this case the DoF is roughly double in favour of FF; ie at 50m the FF will offer a DoF of 2.3m whereas the APS-C will only be 1.25m.

More importantly at the near wicket FF will off a full body shot plus margin for error but the APS-C will only offer about 7/8th of a body. Great for cropping tight but with limitations.

So yep, depending on your circumstances, the crop factor of your camera can make a considerable difference.
 
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Robert
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#21
For everyone saying don't worry about focal length (or more strictly FoV) and DoF equivalents, consider this...


You are at a fixed point and the subject is likewise at a fixed point. Lets take The Kia Oval, the press positions and the wickets as an easy example.

I'm using a Canon 500/4 and have two available bodies for that lens (as well as using one short and one wide lens on other bodies).

One is a 1Dx (FF) the other is a 7DII (APS-C - x1.6).

therefore I can either shoot at an FoV equivalent to 500mm @ f/4 or 800mm at f/5.6.

In this case the DoF is roughly double in favour of FF; ie at 50m the FF will offer a DoF of 2.3m whereas the APS-C will only be 1.25m.

More importantly at the near wicket FF will off a full body shot plus margin for error but the APS-C will only offer about 7/8th of a body. Great for cropping tight but with limitations.

So yep, depending on your circumstances, the crop factor of your camera can make a considerable difference.
or use different systems with same lenses, it is all a bit irrelevant.
So what I stated in post 2 is correct. :)
 
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Robert
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#23
5 words verses probably a thousand or so? I suspect that my post and opinion probably stands up - especially as you didn't elaborate on your comment.
5 words or a thousand, same result. :)
I'm not suggesting your post and opinion don't stand up, I actually agreed with you.
 
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#25
For everyone saying don't worry about focal length (or more strictly FoV) and DoF equivalents, consider this...


You are at a fixed point and the subject is likewise at a fixed point. Lets take The Kia Oval, the press positions and the wickets as an easy example.

I'm using a Canon 500/4 and have two available bodies for that lens (as well as using one short and one wide lens on other bodies).

One is a 1Dx (FF) the other is a 7DII (APS-C - x1.6).

therefore I can either shoot at an FoV equivalent to 500mm @ f/4 or 800mm at f/5.6.

In this case the DoF is roughly double in favour of FF; ie at 50m the FF will offer a DoF of 2.3m whereas the APS-C will only be 1.25m.

More importantly at the near wicket FF will off a full body shot plus margin for error but the APS-C will only offer about 7/8th of a body. Great for cropping tight but with limitations.

So yep, depending on your circumstances, the crop factor of your camera can make a considerable difference.
So you are sitting there with your two cameras and the play is getting exciting (OK, its cricket, but never mind) and you start doing maths and miss the shot. Or you know how your two cameras work and use the camera that will get you the shot. Or more likely, use the camera that already has the lens attached.

Your point seems to be the difference in depth of field between the two cameras but you know that before you leave home so there is still no need for maths.
 
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Andrew
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#26
For everyone saying don't worry about focal length (or more strictly FoV) and DoF equivalents, consider this...


You are at a fixed point and the subject is likewise at a fixed point. Lets take The Kia Oval, the press positions and the wickets as an easy example.

I'm using a Canon 500/4 and have two available bodies for that lens (as well as using one short and one wide lens on other bodies).

One is a 1Dx (FF) the other is a 7DII (APS-C - x1.6).

therefore I can either shoot at an FoV equivalent to 500mm @ f/4 or 800mm at f/5.6.

In this case the DoF is roughly double in favour of FF; ie at 50m the FF will offer a DoF of 2.3m whereas the APS-C will only be 1.25m.

More importantly at the near wicket FF will off a full body shot plus margin for error but the APS-C will only offer about 7/8th of a body. Great for cropping tight but with limitations.

So yep, depending on your circumstances, the crop factor of your camera can make a considerable difference.
Based on your post it sounds like you have actually done this? Care to share the shots for comparison?
 
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Phil
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#27
One of the reasons that some people think you need to take in to account the sensor size is because they are trying to recreate the same photo, for example, you shoot a portrait with a 50mm on a full frame then switch to a crop sensor with a 50mm. In order to get the same composition you will need to move back which means you will need to change the aperture to maintain the depth of field.
That wouldn’t give the same photo.
You can’t zoom with your feet, we have different focal lengths because perspective is a function of subject distance.

So in the real world, when you switch to crop, you’ll switch to a 35mm ish lens, to maintain a similar image. That’ll give a similar subject distance, and you will indeed notice the DoF difference.

I’d need a 1.2 85mm on crop to create the ‘same’ image as f2 135mm on FF, my 1.8 doesn’t manage it.

edit to add...
At 10ft (decent portrait distance) both 1.6 crop 85mm 1.2 and FF 135 f2 give a DoF of 0.19 feet. Those images would be almost identical and shows just over a stop of aperture difference.
 
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Mark
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#28
So you are sitting there with your two cameras and the play is getting exciting (OK, its cricket, but never mind) and you start doing maths and miss the shot. Or you know how your two cameras work and use the camera that will get you the shot. Or more likely, use the camera that already has the lens attached.

Your point seems to be the difference in depth of field between the two cameras but you know that before you leave home so there is still no need for maths.

As you suggest, ou do the calculations before the match starts. But that doesn't mean that you ignore the differences in effect with the two bodies.
 
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