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03-29-2012, 09:14 AM   #1
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Q: about magnification of old lenses on dslr and flashes

I've been wondering about a thing when i comes to the magnification (crop factor) of old lenses on new dslr.
If i'm not completely out of the blue the magnification on my pentax k5 is 1.5.
Which to my understand makes my 50mm lens equal to a 75mm in terms of "magnification/crop factor/field of view" (not sure which term is correct, hope you understand what i mean).

Would this mean that my 55mm lens with 1:1 macro is equal to a 55mm 1.5:1 macro or a 77.5mm 1:1 macro ?
I'm guessing the latest, but would would be nice to be sure if anyone knows.

Another thing i been wondering about is if my focus distance is effected by the "conversion", personally i haven't noticed any difference so i guess it's not. But what i'm wondering is if for example my min focus distance is 20cm on an old lens would it on a dslr have 30cm min focus distance?

Also when using a flash with manual settings, the one i have have a selection of focal lengths, Zoom,24,28,35,50,70,80,105. When using an old lens should i use the focal length that the lens actually have or the "magnification/crop factor/field of view" that it give? The reason i'm wondering is because there is no settings for that.
My guess here is that i should use the settings matching the focal length on the lens and not what it converts to on a dslr but i'm unsure and like a confirmation if i'm doing it correct.

As i've understood it's mostly the field of view that is changed (smaller) when using a old lens on a new dslr camera and everything else is the same. But could that mean that i won't be so much affected with vignetting/soft corners and other similar problems some old lenses had on their originally intended cameras?

That the few things that's been on my mind lately. Hope i'm not to confusing with what i'm trying to say/ask.
And i'm sorry if these things have been answered before but i haven't been able to find a clear answers to my questions so far.

Thank you.

03-29-2012, 09:28 AM - 1 Like   #2
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It's easy to "overthink" this issue. A 55mm lens is always a 55mm lens, and at minimum focus the lens-to-subject distance is the same no matter what camera you have it mounted on. That means a lens that offers 1:1 on film will still offer 1:1 on your dSLR, but the frame won't cover as much. So it you enlarge each to the same size print without cropping, the dSLR with smaller sensor will print the subject larger on the print, because you must enlarge the sensor image more to cover the same print. But 1:1 on a macro refers to the image size on the film (or sensor) to the size of the subject.

If you are using a manual flash with a zoom diffuser, I believe it spreads the light out to cover the field of view of the lens. So if you set it to 50 and use a 50 on your dSLR, the flash will cover more area than the picture will. So what? That setting keeps things simple.
03-29-2012, 11:15 AM - 1 Like   #3
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the question touches on a lot of different points so I'll tackle them one at a time

the K5, and all pentax DSLRs use an ASP-C sensor, and although there are some very minute size differences, for practical purposes you can consider them to have a crop factor of 1.5, as they are all close enough to 16mm high x 24 mm wide that the minor differences do not matter

this means, the field of view of a 50mm lens is equivelent to a 75mm lens on a film SLR. Hold on to this thought, we'll get back to it.

BUT the crop factor does not have any other impact on the lens. when talking "macro" we never discuss the image size on anything other than the sensor, therefore the macro magnification does not change. think of it this way, cut the middle 50% out of a 4x6 print taken on a film camera, whats left is what you would get, printed with the same enlargement out of a DSLR. Nothing else about the lens changes. Not focusing distance, not lens speed not focal length Nothing at all changes with a cropped sensor but the field of view.

Now, add a flash, with a zoom head, the zoom head is marked in focal lengths, and if it is an older flash from a film camera, these focal lengths represent field of view for a lens on film. you need to consider the crop factor when using the flash, if you want to get the optimum reach, so again with a 50mm lens on the camera, and an old flash, you would set toe zoom head to 75mm. This would narrow the dispersion of light down and give you some additional reach, but, even if you do not do this, you hurt nothing. an error in the flash focal length setting just puts some of the light out beyond the field of view of the lens.
03-29-2012, 02:19 PM   #4
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Thank you TomB_tx & Lowell Goudge for taking time to explain to me, i feel more sure about my thought on the matter now.

Am i correct in assuming that vignetting and edge softness might be more limited when using an old ff lens on a dslr with crop factor? What i'm thinking is that if my FoV is smaller then the edges of the lens wont effect the image nor will filters stacked cause problems in the same way as on a ff?

03-29-2012, 02:48 PM   #5
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Yes, since the dSLR only takes the central part of the image circle, there will be less vignetting and better edges. However, you also enlarge the smaller sensor more for the same size print, so it places more demands on the lens to be as sharp as possible.
03-29-2012, 02:55 PM   #6
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Ah, i see. Thank for clearing that up TomB_tx.
03-29-2012, 07:32 PM   #7
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A lens remains the same -- doesn't stretch nor shrink when moved between cameras.

My favorite example. Cut a picture from a magazine. On it, draw a 60x45mm rectangle. Inside that, draw a 36x24mm rectangle. Inside that, draw a 24x18mm rectangle. These are the sizes of the frames for 645 MF, 135 FF, and APS-C cameras respectively. The picture remains the same. Each format's frame sees (crops) different amounts of that picture, that's all.
03-29-2012, 07:44 PM   #8
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QuoteOriginally posted by RioRico Quote
draw a 24x18mm rectangle
24x16, I suppose

03-29-2012, 09:24 PM   #9
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QuoteOriginally posted by sterretje Quote
QuoteOriginally posted by RioRico:
draw a 24x18mm rectangle
24x16, I suppose
25x17 for nominal APS-C.
24x16 for my K20D.
Whatever for a Kr.

Various dSLRs have variously-sized sensors. The format isn't as fixed as 135/FF was. I use 24x18 (135/HF, half-frame) as a close-enough approximation for this exercise. If I were to include a m4/3 or 135/QF (quarter-frame) or 110 rectangle, I'd call it 18x12 even though 110 and nominal m4/3 are 17x13. As my busking partner said as we tuned my resophonic guitar and his tenor sax: close enough for jazz.

And that brings up a funny point. In film days, various cameras might have various frame sizes, but they WERE pretty standardized. 56x56mm for 120/MF, 36x24 for 135/FF, 17x13 for 110, 26x26 for 126, etc. With digital, no such standards -- it depends on what size sensors the silicon foundries churn out. Our 645D does NOT have a 645 (56x41.5mm) sensor -- it's 44x33mm, much smaller than the film standard. So film is/was standardized, and digital ain't. Pretty funny, eh?
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