If I am using a 50mm prime lens, what does the "crops factor" do in terms of what perceived focal length will be?

I know the physical focal length of the lens does not change. The image it produces on the sensor, however, would be approximately equal to what focal length on a full-frame sensor?

If the crop factor is 1.5 (such as on my Nikon D90), are the images I produce roughly equivalent to having a 75mm lens on a full-frame sensor?

Field of view wise, yes.

You have the correct understanding of the percieved image.  Of course there are many that will try to explain in in great detail to impress you with their knowledge.  

For a visual expanation, see this wikimedia image (Creative Commons): http://commons.wikimedia.org/wiki/File:Full-frame_vs_APS-C.svg

Jeez, I typed this whole thing up, then re-read the OP and it didn't mention depth of field once... even though I THOUGHT that was the initial question. Humm... well here's a useless bit of sidetracked info anyhow, since I went through all the effort of typing it.


The easiest way (for me) to comprehend it was to do the following test and compare results:

Find a subject that doesn't move, and has a background that you can see the bokeh.
Take crop camera, 50mm lens and frame the subject, shoot at (for example) f/2.
Take FF camera, 50mm lens and shoot from same spot @ f/2. Your subject will not fill the frame here, but you can compare results.
Take FF camera, 80mm lens (in the case of a 1.6x comparison) and shoot from the same spot @ f/2. Subject should look similar in size to shot #1.
Take FF camera, 50mm lens, and step forward until you have a similar framing/FoV to shot #1.

The results should be along these lines:
Shot 1 and 2 will have the same dept of field. If you crop shot 2 to match shot 1, they should look virtually identical.
Shot 3 will have a shallower depth of field due to the longer lens.
Shot 4 will have a shallower depth of field due to being closer to the subject.

Think of it like this: With a full frame camera you can move closer to the subject and use the same lens. Closer to subject = more shallow DoF.

The inverse is true... if you WANT more DoF, a smaller sensor will allow you to accomplish this without having to stop down.

So no, the 50mm crop shot is not equal (DoF) wise to a 75mm FF shot

Just too funny.

Especially considering I have a (recognized and self admitted) problem with reading too fast, and ASSUMING what I read is what I think it was.

My wife gives me grief over that all the time. I oftentimes don't even read street signs names properly... it doesn't have to be a massive paragraph to screw me up, oh no sir.

Many of the discussions I join in that turn into debates on here are because I can't slow down enough to comprehend what was actually posted in the first place, or thinking I did a good enough point of saying what I meant to, but missing completely.

And just for the record, I couldn't care less whether anyone is impressed or not, I just enjoy learning about how things work, and assume most other people would like to know, too... which is why you won't usually see me commenting (good or bad) on actual photos very often, only on the mechanics side. Although I will readily admit I could be the odd one when it comes to stuff like that... so feel free to ignore my posts if they come across as a diatribe or rant in any way. They're not intended to be so (usually).

Thanks for point this out clearly! I've been trying to figure out how to put this into words as you did.

Join the club, my wife's after me all the time about the same thing.  

I'm glad someone found a use for it... I sat there for 2 minutes debating whether to try and re-write the whole post to make sense, or just delete it. Eventually I gave up the re-write idea, but decided I didn't want to sacrifice everything I typed out... thus the hijack.

Yeah, I'm a geek (hopefully haven't stooped to nerd level)