magicianhisoka
Well-known
Hi all, recently picked up a new 50mm and I'm a little concerned it might be out of whack with my M2 body.
So the way I tested it was by setting the focusing distance to 0.8m on my 35 cron (since that's the MFD of the new 50) and moving back and forth till the focusing patch is aligned. This was done on a tripod btw. I then swapped over to the 50mm and lo and behold, it couldn't focus that close. The distance marker is showing approximately 0.9m.
Is there a potential problem in the new 50mm that I just got? A buddy of mine mentioned that the distance between me and the film plane shouldn't change as a result of different lenses. In other words, standing 0.8m away from an object should be the same regardless of the 35mm and 50mm.
Hope someone can shed some light on this
So the way I tested it was by setting the focusing distance to 0.8m on my 35 cron (since that's the MFD of the new 50) and moving back and forth till the focusing patch is aligned. This was done on a tripod btw. I then swapped over to the 50mm and lo and behold, it couldn't focus that close. The distance marker is showing approximately 0.9m.
Is there a potential problem in the new 50mm that I just got? A buddy of mine mentioned that the distance between me and the film plane shouldn't change as a result of different lenses. In other words, standing 0.8m away from an object should be the same regardless of the 35mm and 50mm.
Hope someone can shed some light on this
maddoc
... likes film again.
The focusing distance scale on every lens is just for guessing the distance, the back focus should be constant independent of the focal length amongst lenses specified for the same camera system.
If you would like to do some testing, you could try the LensAlign tool, or construct something similar yourself...
http://www.bhphotovideo.com/c/produ..._LA_LA2_LensAlign_MkII_Focus_Calibration.html
http://www.bhphotovideo.com/c/produ..._LA_LA2_LensAlign_MkII_Focus_Calibration.html
Landshark
Well-known
The film plane is a fixed distance built into the body. Early ltm cameras used shims to adjust it, but not later cameras.
If the 50 doesn't focus to it's minimum distance it would be a minor task to have it adjusted. More importantly, does it focus correctly at other distances out to infinity?
If the 50 doesn't focus to it's minimum distance it would be a minor task to have it adjusted. More importantly, does it focus correctly at other distances out to infinity?
Monochrom
Well-known
Hi, the only way to test the camera is setting infinite in the lens and seing if the split image matches perfectly a subject in that distance. If it doesn´t it´s off then you´ll notice it at closer ranges much more.
Testing focus point at closer range is not a true test since scales are only a reference.
Every focal length lens will work the same way as mentioned above.
The camera can be ok and still giving misfocus at full aperture and closer range, that will be a matter of focus shift.
Focus shift is a matter of lens design not a flaw occured by handling incorrectly your gear.
Focus shift is a relation between aperture and focus point.
Front focus will be constant, so it´s easier to handle on experience
Back focus will increase as the focus distance increases, usually people think it´s covered by dof, in film it happens if you don´t enlarge, on digital it´s more evident as you pixelpeep!
Of course the lens can be poorly assembled but for bieng sure perform the tests mentioned above.
Cheers!
Testing focus point at closer range is not a true test since scales are only a reference.
Every focal length lens will work the same way as mentioned above.
The camera can be ok and still giving misfocus at full aperture and closer range, that will be a matter of focus shift.
Focus shift is a matter of lens design not a flaw occured by handling incorrectly your gear.
Focus shift is a relation between aperture and focus point.
Front focus will be constant, so it´s easier to handle on experience
Back focus will increase as the focus distance increases, usually people think it´s covered by dof, in film it happens if you don´t enlarge, on digital it´s more evident as you pixelpeep!
Of course the lens can be poorly assembled but for bieng sure perform the tests mentioned above.
Cheers!
Your buddy is right. The film plane is actually the location in the camera adjacent the film rails, at which the film is located, and on which the lens will, hopefully, focus. Different lenses won't change the distance between you and the film plane. (How you hold your camera will, however this is going to be of little relevance here.) And if you're standing .8 metres away from an object, you'll be .8 metres away from it. The issue is, what the scale on the lens says, while you're standing there. Think about it...Hi all, recently picked up a new 50mm and I'm a little concerned it might be out of whack with my M2 body.
So the way I tested it was by setting the focusing distance to 0.8m on my 35 cron (since that's the MFD of the new 50) and moving back and forth till the focusing patch is aligned. This was done on a tripod btw. I then swapped over to the 50mm and lo and behold, it couldn't focus that close. The distance marker is showing approximately 0.9m.
Is there a potential problem in the new 50mm that I just got? A buddy of mine mentioned that the distance between me and the film plane shouldn't change as a result of different lenses. In other words, standing 0.8m away from an object should be the same regardless of the 35mm and 50mm.
Hope someone can shed some light on this
What you are likely referring to is the focus point. If you want to check this at close range for accuracy, the best way to do this, is to use a tape measure to carefully measure the distance from the film plane, NOT the front of the lens, to the focus point. If the distance scale on the lens agrees with the tape, and also, the focus patch coincidence, this is looking good. (Although you really should at least also check the focussed image at the film plane with a loupe and ground glass--more on that in a minute).
I don't particularly agree with this (in bold). If a lens is mounted onto a camera of some quality, that it is designed to fit, with the correct lens register, then, personally, I would not be too impressed if, on focussing to a measured distance, the distance on the focus scale was substantially out. The remainder of your remarks, I quite agree with.
Well, yes and no. It's certainly true that in most cases the master adjustment for the focus of a lens is achieved by adjusting the infinity stop, for instance, and that when correctly set the infinity focus should be good. If you have a collimator and can adjust this accordingly you'll probably be looking fairly good.
For the rest of us who don't have access to a collimator the problem is that the infinity focus of a subject at great distance can look quite good--even very good--however if it is slightly off, there will, as you say, be discrepancies at closer distances.
On the other hand if infinity focus is perfect (nominally) then, a critical check of focus accuracy at close range will also yield good results. When I'm adjusting focus, I do, indeed, set it according to the infinity focus in the first instance. However I invariably check it also at the film plane at minimum focus distance with a tape, ground glass, loupe and similarly, through the viewfinder as well. This last tells me that the viewfinder focus system (SLR mirror; TLR viewing lens; RF focus patch; etc.) is correctly calibrated to the film plane.
If this all comes out OK, I know the infinity adjustment has been done well. If, on the other hand, there is a discrepancy between accuracy of focus at infinity and at close range, re-checking usually reveals the infinity focus is not quite right. So I would always recommend checking focus at infinity and also at or near minimum focus distance, because, short of using a collimator, this is the only way of ensuring that all focus distances in between those two extremes, are actually going to be correct in use.
Of course it is possible for a lens focus at the film plane to be well matched to the viewfinder focus at all distances, and to still display incorrect distance readings on the lens barrel scale at, say, a metre. However it shouldn't be assumed, automatically, that if a cursory check through of the focus patch indicates the lens scale is off that the distance ring is out. It could also be that the lens adjustment is correct and that it is, in fact, the rangefinder focus adjustment (or SLR mirror, etc.) that is the problem. The only sure way to ascertain the situation is to check focus of both film plane and viewfinder focus system at the infinity and close distances and assess the results.
The shorter the focal length is, the harder it is to accurately set the infinity focus without collimation. I learnt this the hard way, when I had to do some work to the 40 millimetre Distagon for my Hasselblad. Yes, I got it right, although that is another story for another day...
Cheers,
Brett