Using old 35mm slr lenses with a DSLR

ash59uk

ash59uk

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Chris
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I have a couple of old lenses for my old Pentax, a 28mm and a 200mm telephoto that I have tried on my Nikon D3100 via a lens converter.

I decided to try them out because my kit lens has a maximum aperture of F5.6 whereas the 200mm has F3.5 and the 28mm F1.8.

Can you tell me what the actual focal length of these lenses will be as I understand that full format and DX cameras give different results with the same lens for example? I have seen a new 28mm for a full format camera that states it will be 42mm with a Dx.
 
Focal length is a physical property of a lens, and is not affected by the camera to which it is attached.

The field of view will differ depending on the camera though. The field of view of a 28mm lens on a DX camera is the same as that of a 42mm lens on an FX camera.
 
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Multiply by 1.5!

So your 200 'effectively' becomes a 300 3.5!
 
StewartR said:
Focal length is a physical property of a lens, and is not affected by the camera to which it is attached.

The field of view will differ depending on the camera though. The field of view of a 28mm lens on a DX camera is the same as that of a 42mm lens on an FX camera.
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I cannot help thinking your reply contradicts itself a bit and a little patronising. It is obvious the lens isn`t going to physically change its self when on a different format camera but the crop factor of the dx that results in the change. I simply wanted to know how to work it out.


Brazo said:
Multiply by 1.5!

So your 200 'effectively' becomes a 300 3.5!
Click to expand...

Oh its that easy to work out.

Many thanks for your straight forward reply.
 
ash59uk said:
I cannot help thinking your reply contradicts itself a bit and a little patronising. It is obvious the lens isn`t going to physically change its self when on a different format camera ...
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I'm sorry if you thought that. But you did say this:
ash59uk said:
Can you tell me what the actual focal length of these lenses will be as I understand that full format and DX cameras give different results with the same lens for example?.
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With respect, it didn't sound to me like it was "obvious" to you. You asked what the "actual focal length" would be.

And there are people who genuinely don't understand this, who think that a 28mm DX lens isn't the same focal length as a 28mm FX lens, and things like that.

But I'm glad we're all singing from the same page.
 
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The whole equivalent thing does excite people as it's rarely uniform when comparing different focal lengths for exposure, depth if field etc but as a Nikon user simplest thing to do is multiply by 1.5 to get the full frame equivalent focal length. Depth if field won't be quite the same. Canon crop users should multiply by 1.6 as they get a smaller sensor!
 
So if I understand this correctly,my 28mm lens when attached to my slr is a wide angled lens but on my Nikon its field of view is narrowed to 42mm due to the crop?

I just want to get some use out of them for portraits as I only have zoom lenses and cannot always get the background out of focus unless I have a bit of distance between the background and subject.
 
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What adapter are you using?

I'm asking because Nikon lenses have a longer distance from the mount to the sensor than Pentax lenses. This means that, for the Pentax lens to work correctly, the adapter will need to have some additional optics. Depending on the design they will give an additional "crop" factor of 1.2 - 1.3, i.e. your 28mm will have an effective field of view equivalent to a 28x1.5x1.2= 50mm lens. Additionally, if it's a cheap adapter, then the extra lenses will almost certainly be of a low quality. I'm not saying to avoid playing with your lenses, just don't have too high hopes for them! If you really want to use them as intended, then look to pick up a cheap Pentax body - all Pentax DSLRs will work with all old Pentax lenses.
 
Cuchulainn said:
If you really want to use them as intended, then look to pick up a cheap Pentax body - all Pentax DSLRs will work with all old Pentax lenses.
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Only if he has K mount lenses. He may well have M42.
 
Cuchulainn said:
What adapter are you using?

I'm asking because Nikon lenses have a longer distance from the mount to the sensor than Pentax lenses. This means that, for the Pentax lens to work correctly, the adapter will need to have some additional optics. Depending on the design they will give an additional "crop" factor of 1.2 - 1.3, i.e. your 28mm will have an effective field of view equivalent to a 28x1.5x1.2= 50mm lens.
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Oh, good point! I'd forgotten that.

So with the adapter the effective crop factor becomes about 1.8 to 2.0 depending on the adapter.
 
It's questionable how usable these will be on a Nikon, but it's all down to the mount (which you don't specify - that sort of information is useful if you want really useful replies, if you don't know posting a shot of the mount would help) - but most will need an optical correction in the mount to work with the Nikon flange distance and that will become the weakest link in the image quality chain (so no skimping on a cheap adapter), otherwise a lens-less adapter will be macro only.


By the way, Stewart answered the question that was asked - but the wrong question was asked, and that was rather obvious (and should have been obvious to Stewart..). Chris, it's really worth understanding what the "crop" in "crop factor" means because it does have implications that can't be understood by thinking that the focal length somehow changes. It's really simple, a smaller sensor sees a smaller section of the image circle projected by the lens. The effect is similar to a child staring down a toilet roll tube and pretending it's a telescope because the field of view has narrowed.
 
Alastair said:
By the way, Stewart answered the question that was asked - but the wrong question was asked, and that was rather obvious (and should have been obvious to Stewart..).
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That's why I answered the question which he asked, and I also answered the question he didn't ask.
 
ash59uk said:
I cannot help thinking your reply contradicts itself a bit and a little patronising. It is obvious the lens isn`t going to physically change its self when on a different format camera but the crop factor of the dx that results in the change. I simply wanted to know how to work it out.
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The answer you received was not patronising, it was correct. The focal length of the lens does not change but the camera just uses a smaller area of it's projected image than a 35mm camera will.

A 28mm lens on a small format sensor is still a 28mm lens but gives an angle of view equivalent to a 42mm lens on a full frame or 35mm camera.

It's better not to think in terms of 35mm coverage though. Just learn what the 'standard' focal length is (about 33mm for your camera) and adjust from there.

When I am using 6x6 medium format cameras, the standard lens is 80mm but I don't think to myself "that's 50mm in 35mm SLR terms" I just know it's standard and that about half that is wide and two or three times that is a long lens.


Steve.
 
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Steve Smith said:
When I am using 6x6 medium format cameras, the standard lens is 80mm but I don't think to myself "that's 50mm in 35mm SLR terms" I just know it's standard and that about half that is wide and two or three times that is a long lens.
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Completely agree with you, however I do find it funny that most EXIF editors include the "35mm equivalent focal length" as an option. I add whatever acquisition data I have to the EXIF of any scans I take and always feel obliged to fill it in with an approximately correct answer :)
 
Thank you to all of you and apologies to Stewart if my question was not clear.
The lenses are k mount. I have an adapter with the correction lens to allow infinity focus. It is a cheapy off eBay but didn't want to spend a lot in case the results were poor.
 
I gave both of my lenses a try out today and the results were pretty poor as the focus was pretty soft probably because the fine focus rings in the center of the viewfinder that are present on my 35mm are absent and of course the lens adapter is a cheapy. In camera they looked ok but not on pc, nor did I achieve the sharp subject, blurred background I was wanting.

A friend of mine has graduated from Uni recently doing photography, so I`ll see if she knows anyone who can make use of them or donate them to the Uni.
 
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