Minimum Focus Distance

[OsmosisStudios]

Very simply: Does minimum focus distance change because of crop factor?

Logically, I would say no. But I've found that my 50mm focuses a bit farther than it should. Minimum on it is 9", but I often have to be a good 12" for it to focus properly. This isn't something new, nor is it a problem, its just something i've noticed.

[Mr Guy]

As a rule, it shouldn't, but I'd not be overly surprised if you didn't run into factor spec collision between the lens and body. It's certainly possible for the lens and body error to both contribute the same way to the over all error, particularly if it's a lens that performs particularly badly wide open.

The usual questions apply, how are you focusing, what are you focusing on, at that distance are you sure it's grabbing the point you think it is, what color/type light are you using to focus, are you using any focal aids (split screen?), etc, etc

[acedrew]

No, minimum focusing distance remains the same, magnification stays the same. It's the laws of optics, has nothing to do with photography. The focal plane for a particular lens is the same regardless of frame size. The only thing that changes is FOV, just because it's a smaller sensor.

Andrew Rodgers
Perfected Perspectives -- Photography by Andrew Rodgers | Andrew Rodgers (acedrew) on Twitter | Login | Facebook

[OsmosisStudios]

This is on a stock D80, outdoors using a single centre-spot AF focusing on a hard edge. It just feels like im farther away then I should be.

[Mr Guy]

Feels as in perspective, or feels as in taking a ruler and measuring, after accounting for things like manual focus, the AF point maybe not being exactly where it ought to be, lighting conditions causing front focusing, misaligned sensors, perspective in the eyepiece being different, etc?

[OsmosisStudios]

I've yet to measure it, but the few times that i've been butting up against the minimum focus distance, i've taken my eye off the viewfinder and found myself to have about 9" from the front of the lens to the subject, not from the sensor plane.

I could be crazy, though

[acedrew]

An easy way to check this is to determine what magnification your lens is, then take a picture of a ruler at minimum focus distance. Use your sensor size x magnification, to determine what size a given dimension on the ruler should be. There may be something wrong with the focusing mechanism on your lens.

Andrew Rodgers
Perfected Perspectives -- Photography by Andrew Rodgers | Andrew Rodgers (acedrew) on Twitter | Login | Facebook

[OsmosisStudios]

Originally Posted by acedrew

There may be something wrong with the focusing mechanism on your lens.

I havent ruled this out either: I've dropped it recently and the focus mechanism got stiffer.

[acedrew]

Theoretically, if there were a difference between crop and full frame, the sensor plane would be in a different position, in which case, the lens would not be able to focus to infinity. It actually would be great if they could be different, then you could make a crop (smaller) sensor, with the same FOV of a larger sensor. I wouldn't be surprised if during the drop your focus ring skipped a few teeth or something. Does the lens have an auto slip clutch or is it an older one?

Andrew Rodgers
Perfected Perspectives -- Photography by Andrew Rodgers | Andrew Rodgers (acedrew) on Twitter | Login | Facebook

[OsmosisStudios]

It's an AF-D, so it's all mechanical. It still focuses accurately, its just stiffer than before.