From what I understand prime lenses give better IQ and are cheaper than zoomlenses. Is that correct? Also zoomlenses will give best results in the middle of their range so theoretically a Tamron 28-75 would give it's best results at 51.5 mm. If I were to use a prime, say 50mm and wanted the subject to cover the same amount in the viewfinder as the Tamron at 28mm, how far back would I have to walk and the other way around? Is there a formula to work out what distance one would have to move bacwards or forwards in relation to the focal lenght of the lens? Does it make a difference whether one uses a crop or FF sensor? I use a 40D Waiting with bated breath. Thanks, Alan

In the old days primes were considered better. Now though with modern technology they are about the same as some zoom lenses. Where the differences are is between "consumer" and "pro" lenses. While most of those differences seem to be in built the one important difference is quality control. For as liitle as 100 dollars you could get that 50mm 1.8 Canon lens that will blow you away. I can't answer the walking back part, that would take some experimentation. It will matter if the sensor is full frame or not. Bottom line from me, if you're going to have one prime make it the 50mm 1.8.

It's pretty much trig & field of view, irregardless of sensor size, if one sticks with that same camera then there is a ratio. Example - allow us to say we work with a 50mm lens on a 35mm film or FF sensor camera and call the 'standard' distance to subject to be 10 feet. That field of view in the viewfinder to be equal at other focal lengths implies 'zooming with yer feets'. The equivalent views will be given for the following focal length - subject distance (as the 10 feet with the 50mm lens) ... 20mm lens = 4.0 feet, 24mm = 4.8 feet, 28mm = 5.6 feet, 35mm = 7.0 feet, (50mm = 10 feet), 85 mm = 17 feet, 105 mm = 21 feet, 135mm = 27 feet and 200mm = 40 feet. Graph this up on a pieece of graph paper or with Excel and you have a handy nomagram to use in the field. The RATIO of let's say 200 mm to 50mm is the same, FF or APS (i.e., 40ft : 10 feet, or 4 to 1). Silly example, you have a scene on a 50mm lensed APS camera let's say 20 feet from the subject. To get the same field of view with a 20mm lens, you have to go to 4/10ths the distance, i.e. 8 feet to 'see' the same 'scene' in your viewfinder. Hope this is clear. Jim M.

Tamron 28-75/2.8 is a very good lens, I wouldn't worry about the sharpness. Prime lenses give you speed, you can't have faster than f2.8 zoom but something like 50/1.4 is common and not that expensive. Do you have the Tamron (or any zoom)? Set it to 28mm, frame, set 50mm and frame with your feet. Should be quite obvious.

Hi Jim, that is rather usefull and yes this is pretty clear. Thanks for giving me your time and brain. Cheers,Alan

But I figure that zooming with your feet just won't work all the time. What if the subject is 500 ft away. Then 50mm/500ft as 85mm/x. X=850ft. So then you have to zoom with your feet a whopping 350ft. What if that puts you in the middle of a lake or the freeway. But I have found that even when I have a zoom, I still wind up zooming with my feet. But it's always fun. Better than pulling weeds.

There is a fundamental difference between "zooming with feet" and changing focal length. Zooming with feet alters perspective while changing focal length affects the crop.

I would say that zooming in or using a telephoto changes perspective from what would appear to be normal. Shooting a sidewalk scene with 400mm and the people all seem to be bunched on top of one another. (A technique that film makers like use to show how crowded city streets are.) Get eight times closer with a 50mm lens and the people are not crowded on top of one another at all.

Yes, of course, the perspective is different. But in the case of my response, it was directed merely for the 2D (Width x Ht) view in the viewfinder. No consideration was made to account for perspective, i.e. elongation with wides nor compression with telephotos and the resultant DOF. There are many ways to skin the proverbial cat. Teles are essential for skittish animals. Wides draw a person into the landscape shot. Taking a shot of a lighthouse from a particular location which cannot be altered demands a certain focal length to give a certain image. A zoom in that situation, is worth a whole bagful of primes, perhaps. Ask a guy shooting a game from the sideline, 'is it easier to turn a zoom or walk 30 feet?' Isn't It all relative after all? Jim M.

"Is there a formula to work out what distance one would have to move backwards or forwards in relation to the focal lenght of the lens?" No. No amount of moving will duplicate a change of focal length. No change in focal length will duplicate a change in distance. Changing distance changes perspective, changing focal length does not. If you don't believe me, try it out for yourself.

In simple terms, just comparing the focal lengths as a ratio will give the distance to move that will leave a dominant subject the same size. So if you have a 50mm lens and a 100mm lens, you would need to be half the distance to the subject with the 50mm to get the same size image on the sensor/frame. For example, a single flower blossom at 2 feet or 4 feet. But as others have pointed out, the perspective will change. How noticeably depends on various factors, especially but not only the amount of foreground and background detail. These two images actually were zoomed with my feet, quite a while back with a digicam. I started with a long setting then approached zoomed wide until the sign seemed to be the same height in the finder. While there are changes to the tree behind the sign, the real differences can be seen in the more distant details. http://www.pbase.com/gillettecraig/image/32576430 http://www.pbase.com/gillettecraig/image/32576432