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Thanks to all you guys for the positive words, and for the encouragement.
DuQues...... Now there is a master! That does indeed make it much easier, thanks for that input - that puts in in good perspective.

I will remember to use that in furture, as it really does make it easier.

To everyone else, remember this, we all need to learn, and I need you guys to jump in with your two cents worth all the time. I know I said I thought we could all do with a redo of the basics, but questions will also be answered. I'm also still learning, and will hopefully do so until the Lord folds me up and puts me away.

I've just come across this very useful site: Photographr.info
Its a question-and-answer site for photography. (The engine is based on a software development Q&A product)

Great link.... some cool stuff

yup, and there are plenty of further links there too

Hi guys. Another beginner photographer recently made a comment in another threadabout focal lengths of lenses changing depending on what body (camera) the lens is on. Since there hasn't been anything on this topic for a while, I thought I'd write a post on the issue.

Please note that I am far from an expert on this issue, but I do feel like I can understand it and explain it in person. If you find a mistake in my post or disagree with me, please write a post by all means.

Instead of writing my own post on the issue, I decided to do some searching to find out how other people had explained it (and to solidify my understanding of the topic). I managed to find some really good articles on it, which explain probably better than I could.

While reading through the links, don't be put off by the maths and the geometry, you will be able to work it out eventually (though doing geometric optics in a university physics course does help).

So here goes:

Probably the best explanation I've seen, including geometry

Written for Canon lenses

Written from a Pentax point of view

The bottom line is that one should think in terms of field of view (or angle of view), and not in focal length. Focal length is a physical property of the lens, whereas field of view is a function of sensor size (and image circle size). I.e. a 50mm lens has a focal length of 50mm, regardless of which body you put it on.

I don't know what the terminology for other makes is, but in Nikon parlance, DX bodies and lenses have an APS-C sized sensor, which is a crop of a 35mm frame.

Nikon FX bodies have 35mm-sized sensors, i.e. the same as a 'regular' film camera. If you put a DX lens on an FX body, you will see dark edges on your frame (known as vignetting). This is because the image circle produced by a DX lens is smaller than an FX sensors. Note: an 'image circle' is the picture produced by the lens that falls onto the image sensor.

Because a DX lens needs to produce a smaller image circle than an FX lens, because the sensor is smaller, DX lenses can be physically smaller than an FX lens, for the same focal length. Thats why a 300mm DX lens is physically smaller (narrower and shorter) than a 300mm FX lens.

You will also notice that a lot of compact and bridge (i.e. non DSLR) cameras have an 'equivalent' focal length marking. This does not mean that the focal length is different, it means the field of view is different. The lenses of compacts and bridges are much much smaller than SLR lenses for the same focal length.

This is again because the sensors in such cameras are much smaller than even the APS-C format. (Smaller lenses generally mean lower quality, more noise, more grain etc, because of the electronics.) (Note I am not talking about megapixel count here, only physical size).

For example my Panasonic Lumix DMC-TZ1 has a 10x optical zoom (35-350mm equivalent) lens which is physically much smaller than the equivalent DX or FX lens would be (taking into account the fact that the Lumix has a a 'folded lens').

So if you really want to sound smart while talking to your photography buddies, rather talk about field of view instead of focal length

I have a question. I have an IS Canon lens (75-300)and a Canon 40D, when I use a tripod or a window mount and all is steady, is it better to turn off the IS or does that not make a difference?

Good questions, best check the manual.
There are several versions of IS around, some 100-400's should be turned off, some not...

Experiment a bit, get some empirical evidence

Will do! I am going to WCNP this weekend.

lucky you. Enjoy!

generally the older style IS/VR was required to be turned off (it needed the movement otherwise it got confused) when tripod mounted the new generation do not apparently (never tried generally when me cam is mounted IS/VR is off)

cheers
Mike

Was my post on crop factors and equivalent focal lengths off the mark?

Mods, should this thread be moved to the new 'Sanparks Camera Club' section?

A good question which was expected
Maybe best to move all tech and learning topics to the camera club
Will get the view from the other moddies.

And what do the forumites think?

Will be a tedious process, but I think it should be done. I'll volunteer to go through some old topics and flag those which I think should be moved.