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06-16-2010, 04:28 AM   #1
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older lenses and image iq on a dslr?

Hi all
I am not sure I really understand this.......if I use any lens LR tells me the 35mm equivelent....
So all lenses seem longer on a dslr? Something to do with the sensor? Does this change the IQ of the lense? Especialy if I was looking for an older manual lens of the 250mm to 300mm?

Am I making any sense at all?
cheers
Jan

06-16-2010, 05:20 AM   #2
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Hi,

first things first : the lens does not change in any way dependant on the body on which it is mounted. That's important to understand.

When you mount it on an APS-C DSLR, the sensor is smaller than a 35 mm film. What that means is:

1-only the central area of the lens is used, meaning you usually use the best part of the lens

2-the image will look cropped when compared to what you would get on film. It's like using a smaller film format, if you will.

In other words, the FIELD OF VIEW is not the same between 35 mm film and APS-C digital. The field of view is smaller on digital.

People usually refer to a "crop factor" (with a value of 1.5 for most brands) and calculate the equivalent focal length. A 50 mm lens gives you the field of view of a 75 mm on film, for instance.

This is misleading, however, since the focal length does not change, nor does the depth of field or related properties of the lens. A 50 mm is ALWAYS a 50 mm (and its aperture does not change either). But it's more convenient to use eqwuivalent focal length, since all photographers understand this, than field of view, which most photographers have a hard time quantifying.

I hope it helps.
06-16-2010, 05:28 AM   #3
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QuoteOriginally posted by ricstew Quote
Hi all
I am not sure I really understand this.......if I use any lens LR tells me the 35mm equivelent....
So all lenses seem longer on a dslr? Something to do with the sensor? Does this change the IQ of the lense? Especialy if I was looking for an older manual lens of the 250mm to 300mm?

Am I making any sense at all?
cheers
Jan
I always get the physical focal length of the lenses, when I use LightRoom. Ofcourse the EXIF data contain both values, the real focal length and the "crop-factor corrected" longer equivalent focal length.

But the physical properties of a lens are fixed (focal length, maximum aperture, minimum focusing distance etc.) and won't change with the camera mounted to the lens. Just as bdery wrote, the only thing that changes is the size of the camera sensor and thus the field of view.

Ben
06-16-2010, 10:42 AM   #4
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If you're already familiar with how focal lengths behave on 35mm film, then Google the term "crop factor" and be prepared to spend a few hours reading before hopefully things become clearer. To get a lens with a similar field of view as a 300mm lens on 35mm film, you'd need a 200mm lens on APS-C digital.

If you're not already familiar with how focal lengths behave on 35mm film, forget you ever heard about crop factors or 35mm equivalent. That's like contantly worrying about what speed you are driving in KPH is you've only ever driven in countries that use MPH. Who cares what the speed would be in some country you've never driven in? Similar, who cares what focal length would look similar on a camera you've never shot with?

Either way, though, IQ doesn't change.

06-16-2010, 01:47 PM   #5
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And ricstew, remember, beware the Ricoh pin. Beware the K-R mount lenses.

theatre of noise: Ricoh Lenses On Pentax Cameras -- The Ricoh Pin Fix
06-19-2010, 09:53 AM   #6
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QuoteOriginally posted by Nowhere Matt Quote
And ricstew, remember, beware the Ricoh pin. Beware the K-R mount lenses.

theatre of noise: Ricoh Lenses On Pentax Cameras -- The Ricoh Pin Fix
Thanks for posting that link - it explained/solved a problem that I had back when I bought my *istD.

At the time, the only lens that I owned with an "A" setting on the aperture ring was a Craig Optics 28-70. It wouldn't mount because of the flange on the back (next to the coupling lever). I removed the flange and modified it to fit. Even then, the lens would only turn when it wasn't in the "A" position. Once mounted, it could be changed, but the shutter would never trip. So I had to go back to where I bought my *istD and "upgrade" to the kit with the 17-35 to get a lens that would work.

Fast forward 6 years (and 5 minutes with a set of Wiha jeweler's screwdrivers to do the lens surgery) and now it works fine.
06-19-2010, 10:12 AM   #7
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I don't think anyone has answered one of the main questions from the OP:

IQ has nothing to do with it. That just stands for Image Quality.
06-19-2010, 02:01 PM   #8
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Original Poster
Ok I think I got it........wow is there some reading out there!
As I see it......... it s not the focal length that changes but the FOV.......so the digital sensor actually " sees" a different ( smaller in my case ) FOV to film........so that it 'seems' a longer lens if you are used to 35mm.
And it doesnt affect IQ........which I know stands for image quality!
Thanks for your help guys.
cheers
Jan

06-19-2010, 03:28 PM   #9
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QuoteOriginally posted by ricstew Quote
Ok I think I got it........wow is there some reading out there!
As I see it......... it s not the focal length that changes but the FOV.......so the digital sensor actually " sees" a different ( smaller in my case ) FOV to film........so that it 'seems' a longer lens if you are used to 35mm.
And it doesnt affect IQ........which I know stands for image quality!
Thanks for your help guys.
cheers
Jan
You got it!!!

And keep in mind what bdery said above, his #2 point:

All lenses are sharper in the center, and since you get a different/tighter FOV on your smaller sensor, the older lenses made for 35mm SLR give you the sharpest/best part of the lens's capabilities.

In other words, when you hear about a lens having a falling out of sharpness around the edges, or being soft around the edges, this doesn't apply to a full frame lens made for 35mm film cameras used on your APS-C, because you're not using that part of the image at all.

It's lost/clipped out because of the DSLR's crop factor.
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