[baj]

I just bought a Pentax Auto Bellows-A with slide copier. My understanding is that my 50mm camera lens won't work for slide copying on a K-5 because the minimum magnification will be too high (need 0.67x magnification to get the whole slide).

Will an EL Nikkor 50mm f/2.8 work for this? I understand how to calculate magnification based on extension, but I don't know how much "extension" (or the opposite thereof) is built into the mounting flange of enlarger lenses. These lenses are cheaper than longer enlarger lenses (probably because they were used for 35mm and are therefore more common) so I'd rather use a 50mm if I can.

Thanks for any advice!

[SOldBear]

I think you need a lens with 70mm focal length. 50mm won't work.

[civiletti]

Why would a longer lens be preferable? That would require more extension to reach the needed magnification.

[baj]

I think you need a lens with 70mm focal length. 50mm won't work.

Thanks (but bummer). Do you (or does anyone else) know the effective "extension" of different enlarger lenses? I'd rather not do a lot of experimentation (by which I mean buying stuff I can't ultimately use).

Why would a longer lens be preferable? That would require more extension to reach the needed magnification.

The problem is that, with a shorter lens, the minimum extension with the bellows collapsed all the way provides too much extension. If the minimum magnification is too high, I won't be able to get the entire 35mm frame onto my sensor (it will be a "crop" sensor in the truest sense I guess).

According to the instructions (pdf here: Pentax auto slide bellows A, Pentax Slide copier A instruction manual, user manual) the minimum extension with the lens mounted in the normal orientation is 38mm. This gives a minimum magnification with a _camera_ lens of 0.76x, which is too much. A longer lens would work (but I realize that a non-macro camera lens might not be the best for copying slides/film). The image quality will be better with the lens reversed, but then the minimum magnification is even higher.

If a 50mm enlarger lens came to a focus further from the mounting flange than a Pentax camera lens then it could work (for instance Pentax, Nikon, and Canon 50mm lenses will all focus infinity at a different distance from their mounting flanges, since each camera brand has a different distance from mounting flange to film/sensor. This is why you can use a Nikon lens on a Canon camera but not the other way around). That was the basis for my question.

[SOldBear]

Why would a longer lens be preferable? That would require more extension to reach the needed magnification.

The APS-C sensor is smaller than 35mm film. You want less magnification, not greater.

[newarts]

A 75mm el nikkor enlarger lens will likely work for you. Its rear principal plane is about 13mm from its mounting surface, so when mounted on your 38mm (min) bellows, it is 45.5(camera) +38(bellows) + 13(lens) = 96.5mm(total). With a 75mm lens the magnification is 96.5/75 -1 = 0.29x Extending the bellows to about 67mm will result in a magnification of about 0.67x.

Dave in Iowa

PS an enlarger lens does a good job with imaging flat objects - that's what it is designed for.

[Not a Number]

I don't have the specifications for the Pentax auto-bellows, but with the Minolta Auto-Bellows III and slide copier from the charts in Minolta's manual a Minolta 85mm lens would give me the magnification needed to fit a full 35mm slide frame on the APS-C sensor in my K10D. I tried a 75mm enlarger lens - bellows rail isn't long enough by ~6 cm (max extension 19.8 cm). The 75mm enlarging lens (a budget El-Omegar) simply doesn't focus close enough to begin with. Remember you have to base your calculations on the actual minimum focus distance of the lens which can be far greater than the focal length of the lens.

85mm lenses MF lenses are not cheap on the used market and I've not found an 85mm enlarger lens which probably has too long of a minimum focus distance to begin with.

Maybe reversing the lens would work for you, it doesn't on my Minolta. Minolta had charts for normal and reversed lenses.

[baj]

If I have to go longer than 50mm, I was thinking about the EL Nikkor 80mm, since it has 6 elements (the 75mm only has 4). I read somewhere on here an 80mm lens _should_ work (can't find the thread now). How's the image quality with the 75mm?

Eventually I'd like to use the bellows for macro as well. For that, I think I'd prefer to use a longer lens (105 or 135mm). Would the 105mm EL Nikkor work, or is that too long for the slide copier? The 105mm has another nice feature: it's easy to reverse mount because it has 34mm female threads and 39mm male threads on the front (at least this version did: Nikon 105mm f/5.6 enlarging lens review), compared to the 80mm's oddball 34.5mm female threads, so it's much more attractive to me for macro than the 80mm. I don't know about other brands of enlarger lenses; I haven't been able to easily find details like this about them.

Just trying to buy as few lenses as possible to do what I want to do.

[newarts]

If I have to go longer than 50mm, I was thinking about the EL Nikkor 80mm, since it has 6 elements (the 75mm only has 4). I read somewhere on here an 80mm lens _should_ work (can't find the thread now). How's the image quality with the 75mm?

Eventually I'd like to use the bellows for macro as well. For that, I think I'd prefer to use a longer lens (105 or 135mm). Would the 105mm EL Nikkor work, or is that too long for the slide copier? The 105mm has another nice feature: it's easy to reverse mount because it has 34mm female threads and 39mm male threads on the front (at least this version did: Nikon 105mm f/5.6 enlarging lens review), compared to the 80mm's oddball 34.5mm female threads, so it's much more attractive to me for macro than the 80mm. I don't know about other brands of enlarger lenses; I haven't been able to easily find details like this about them.

Just trying to buy as few lenses as possible to do what I want to do.

The outline I gave for lens choice (regardless of quality) is valid:

f = (45.46 + bellows extension + distance from lens mounting surface to rear principal plane)/(mag+1)

If you use a type K lens, this is:

bellows.extension = f(m) = 0.67f, so a 75mm lens requires 50mm extension.

Dave in Iowa

[baj]

Thanks. I understand the relationship between magnification and extension. What I don't know is the distance from the mounting surface to the rear principal plane (or the flange focal distance) for enlarger lenses (and I realize that it may not be consistent in any way between different lenses). I was hoping to get some real-world "this lens works, this one doesn't" kind of experiences.

I get that the idea of flange focal distance doesn't exactly apply to enlarger lenses in their intended application. But the concept is still valid: An object at infinity comes to focus a distance behind the rear principal point that's equal to the focal length, but I don't know (except for the one lens that newarts kindly provided) the distance from the mounting flange to the focal plane.

I'm sure I can get 0.67x with a 105mm enlarger lens. The only question would be if the bellows and slide copier rails are long enough.

[newarts]

I have a Russian 110mm enlarger lens and like all enlarger lenses is roughly symmetrical. This implies the principal planes will be close to the physical center of the lens.

This implies about 15mm flange to principal plane distance (inside the lens). so for 0.67x,

0.67 +1 = (45.5+bellows+15)/110 or bellows = 1.67*110 - 60.5 = 123.5mm.

My bellows (140mm) might just make it!

[baj]

My bellows (140mm) might just make it!

What kind of bellows do you have? The one I bought is supposed to go to 170mm (but at this point all I know about it is the instructions I downloaded) .

[newarts]

What kind of bellows do you have? The one I bought is supposed to go to 170mm (but at this point all I know about it is the instructions I downloaded) .

It is an inexpensive ~ $50USD PK type from Fotodiox IIRC.

Dave

[baj]

My reference to the EL Nikkor 80mm having an oddball 34.5mm filter thread apparently applies only to the older-style lenses without illuminated aperture scale. The newer N-type 80mm lenses have 40.5mm filter threads, which is easy to find in step-up rings. All of this information comes from scanned Nikon brochures that I found online, so assuming I'm interpreting the drawings correctly it should be accurate (as things you read on the internet go, anyway). The old-style 75mm and 50mm f/4 (but not the 2.8) have the same 34.5mm thread.

I realized that the old-style 80mm won't just be problematic when reversed; even when mounted forward for slide copying it's necessary to attach the front of the lens to the slide copier bellows. So unless it's only going to be used for forward-mounted macro (without the slide copier), or you don't mind duct tape solutions, or you have a very rare Nikon adapter ring laying around, or you are friends with a machinist, it seems best to avoid the old-style 80mm EL Nikkor.

[JimJohnson]

I found 70mm to be the sweet spot along with a couple extension tubes on filter side of the lens to move the slide further away. See my rig with photos in this thread:
https://www.pentaxforums.com/forums/pentax-camera-field-accessories/170530-ho...ml#post1765549