ISO FF and DX

ThomasH

ThomasH

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I never really was in to the tech side of photography so please forgive the possibly ridiculous question. At first I was really impressed with my A7s ability to shoot with such low noise at ISO 6400. But then started to wonder am I just being mislead by some mathematical magic that is going on?

I know that when swapping from a full frame body to a full frame body there are many defences.

For example I know that a 28mm lens on a full frame camera could be the ruff equivalent of a 18mm on a APS-C camera if we are talking effective focal length.

I also know that DOF is also effected is such a way. So I know to get the same DOF as the 28mm lens on the full frame camera if it was f2.8 the 18mm lens on the APS-C camera would have to be f1.8.

I also know that the overall light gathering ability of both of these lenses would be the same again approximately speaking. So if you had a 100% coverage viewfinder the brightness would be the same.

So to my question, if both the full frame and APS-C cameras with the above lenses set at the same ISO and the same shutter speed would they get the results? Or are iso and/or shutter speed effected is a similar way as dof, effective focal length and light gathering ability?

Thanks
 
Shutter speed and ISO would be the same (ignoring any differences in the metering system). In essence the exposure parameters stay the same, it is the sensor size that affects the other things like DoF and FoV.
 
Nawty said:
Shutter speed and ISO would be the same (ignoring any differences in the metering system). In essence the exposure parameters stay the same, it is the sensor size that affects the other things like DoF and FoV.
Click to expand...

I think it's rather that the sensor size influences your choice of lens/focal length and /or camera to subject distance and thus affects other things.

For example for the same FoV if you're using a 50mm lens on FF at f8 and a 25mm lens at f8 on Micro Four Thirds (I specify MFT as it's x2 crop and it keeps the mental arithmetic easy :D the exposure would be the same but the DoF would be deeper with MFT as you're using a wider lens with a smaller aperture. If you equalise the aperture, 50/8 = same as 25/4, and shoot MFT at f4 the DoF will look the same.

ThomasH said:
So to my question, if both the full frame and APS-C cameras with the above lenses set at the same ISO and the same shutter speed would they get the results? Or are iso and/or shutter speed effected is a similar way as dof, effective focal length and light gathering ability?
Click to expand...

The A7s has big fat pixels. That helps low light performance.
 
Thanks for the link looks like I could have a long read ahead of me :)

Brain melting :wacky:
 
You're nearly there Thomas ;) For exact theoretical equivalence, with same generation sensor with same pixel count, and using a 1.4x crop factor to make the maths easier:

Full-frame
50mm lens
1/125sec
f/8
iso200

Same as APS-C 1.4x
36mm lens
1/125sec
f/5.6
iso100

With same subject taken from same position, that would give same:
Framing and perspective
Control of movement blur
depth-of-field
noise

The only thing that would then be different is sharpness, due to the higher resolution demands of the smaller sensor. In round figures, the APS-C lens would need to have about 10-15% higher MTF performance to equalise the visual difference.
 
where ff helps I guess is when your iso limited, cant drop f stop to compinsate, or shutter speed, there's few zoom lenses or primes which can bridge the gap, often u can get same speed from ff
 
Here are a couple of videos which discuss the effect or crop factors on other accidents of a camera - ISO, focal length, aperture, ...


Unfortunately, the presenters do not speak the king's English, but rather some mangled version.

But at least you do not need to read.
 
HoppyUK said:
You're nearly there Thomas ;) For exact theoretical equivalence, with same generation sensor with same pixel count, and using a 1.4x crop factor to make the maths easier:

Full-frame
50mm lens
1/125sec
f/8
iso200

Same as APS-C 1.4x
36mm lens
1/125sec
f/5.6
iso100

With same subject taken from same position, that would give same:
Framing and perspective
Control of movement blur
depth-of-field
noise

The only thing that would then be different is sharpness, due to the higher resolution demands of the smaller sensor. In round figures, the APS-C lens would need to have about 10-15% higher MTF performance to equalise the visual difference.
Click to expand...
This ought to be chiselled in stone somewhere. Then every time someone starts to ask the same questions about FF vs crop again, we could get the stone and throw it at them.
 
I hope not. The math doesn't even add up.

Theory is nice and more so if the numbers work and relate to reality but they can also mislead a bit. My worry with this example is noise and at what image size it would be significant in the APS-C image given identical settings. I'd guess that on most images there'd be little if any detectable difference at low ISO yet the numbers imply that there would be hence the different settings.

I think that it's best to test this for yourself if at all possible and not simply rely on numbers. If relying on numbers we could form the impression that smaller formats are noisy and even that they need different exposure settings but in actual use and image viewing there may be very little difference between a FF and smaller format image and indeed some smaller format cameras may arguably outperform some FF ones.
 
woof woof said:
I hope not. The math doesn't even add up. <snip>
Click to expand...

What math doesn't add up? Mine does.

It's a theoretical comparison, but it does actually stack up in practise if you can find the right cameras. That's not easy, but two good examples are the Canon 5D2 vs Canon 7D (similar pixel count, similar generation tech) and Nikon D700 vs D300 (ditto).

But that still leaves the problem of finding an APS-C lens that is 10-15% sharper than a full-frame equivalent, and I don't know that such a beast exists. Even though it is easier to make sharper lenses for smaller formats, that gap is too large to bridge. Which is why full-frame always gives superior image quality.

Whether or not the difference is significant or meaningful or whatever is another question, though it's certainly there.
 
It doesn't exactly add up and who uses a x1.4 crop camera? Maybe I've just missed them but I thought that the common crops were x1.3, if they still make them, x1.5, x1.6 and x2.

Anyway. I've read some strange things on line about "crop cameras" and personally I think that a note of caution is called for when appearing to imply that exposure settings may vary even if only in the mind of a reader who may misread.

As in...

"Full-frame
50mm lens".... etc.

The exposure remains the same when changing formats. f8 is f8. There'll only be differences due to what Canon / Nikon or whoever think should be ISO xxx.

I'm also more than a little nervous talking about noise and appearing to imply that APS-C will produce similar noise at ISO 100 to a FF at ISO 200 as I think that it's possible that any noise difference will be next to invisible at these ISO's in the vast majority of "normal" images, and by normal I mean at normal image sizes and viewing distances for these days.

I know that talking about equivalence on line is dangerous and often ends badly :D and although the theory may well be sound in practice I do wonder how much the finer points matter. To me what matters most is getting the picture and acceptable image quality, output size and viewing circumstance and those things dictate my kit and settings. Equivalence wise what matters to me is field of view and also depth of field and leading on from those things shutter speed and ISO setting. The former are easy to relate to crop factor and the latter could well be an advantage of smaller format systems especially for hand held shooting. ie. I can get pretty much the same image I'd get with a FF camera with a x2 crop camera but at a wider aperture and faster shutter speed and that could matter to me. What matters relatively less to me equivalence wise are maximum output size, noise and the finer theoretical points of CoC.

As I said, personally I think that noise wise and indeed on the wider subject of equivalence it's best to try side by side comparisons and decide for yourself. Personally I think that the issue of noise is overrated when talking about equivalence and maybe image quality is a little too laboured too.

Sorry about the big text last time. Don't know how or why... and I'm sure I couldn't do it again :D
 
Last edited:
woof woof said:
It doesn't exactly add up and who uses a x1.4 crop camera? Maybe I've just missed them but I thought that the common crops were x1.3, if they still make them, x1.5, x1.6 and x2.

Anyway. I've read some strange things on line about "crop cameras" and personally I think that a note of caution is called for when appearing to imply that exposure settings may vary even if only in the mind of a reader who may misread.

As in...

"Full-frame
50mm lens".... etc.

The exposure remains the same when changing formats. f8 is f8. There'll only be differences due to what Canon / Nikon or whoever think should be ISO xxx.

I'm also more than a little nervous talking about noise and appearing to imply that APS-C will produce similar noise at ISO 100 to a FF at ISO 200 as I think that it's possible that any noise difference will be next to invisible at these ISO's in the vast majority of "normal" images, and by normal I mean at normal image sizes and viewing distances for these days.

I know that talking about equivalence on line is dangerous and often ends badly :D and although the theory may well be sound in practice I do wonder how much the finer points matter. To me what matters most is getting the picture and acceptable image quality, output size and viewing circumstance and those things dictate my kit and settings. Equivalence wise what matters to me is field of view and also depth of field and leading on from those things shutter speed and ISO setting. The former are easy to relate to crop factor and the latter could well be an advantage of smaller format systems especially for hand held shooting. ie. I can get pretty much the same image I'd get with a FF camera with a x2 crop camera but at a wider aperture and faster shutter speed and that could matter to me. What matters relatively less to me equivalence wise are maximum output size, noise and the finer theoretical points of CoC.

As I said, personally I think that noise wise and indeed on the wider subject of equivalence it's best to try side by side comparisons and decide for yourself. Personally I think that the issue of noise is overrated when talking about equivalence and maybe image quality is a little too laboured too.

Sorry about the big text last time. Don't know how or why... and I'm sure I couldn't do it again :D
Click to expand...

The maths adds up. I used 1.4x as an example because it makes the difference easier to see, ie there is exactly one stop difference between full-frame and a (hypothetical) 1.4x crop format camera. If you want to use 1.5x or 1.6x, then do the maths yourself - you'll end up with numbers like f/5.333-recurring. If comparing ISO200 and ISO100 is unconvincing in your view, then substitute ISO3200 and ISO1600 - same difference.

The whole equivalence debate is hypothetical, if only because there are simply no directly/exactly comparable cameras made. But there are examples that are very close, as given above, and they demonstrate the theory more than adequately.

ps Exposure and equivalence are not the same thing. Equivalence article from DPReview http://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care
 
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