Can I get better image quality with a higher focal length or by cropping an image from a sharper lens?

Question

Two well known ways to get better image quality are to use a sharper lens, and to zoom in to fill the frame (as opposed to zooming out and cropping in post). Sometimes there is a give and take between these two.

I'm considering buying a slide copier that attaches to the front of your lens to let you take high quality digital images of your 35mm film slides and negatives. The product website says that a 100mm focal length (35mm equivalent) is ideal because it will fill the frame with the image of the slides/negatives.

I have a 1.5x crop factor on my camera, so that would be a 66mm focal length. That leaves me with a choice of two lenses: My 55-300mm zoom lens, or my 50mm fixed focal length lens. The 50mm lens is sharper, but I would have to crop the resulting image.

So how do I make an informed decision between the two? Is there some numerical representation of the sharpness of the lens that I could use? (If I had data on each lens?) Or is there some rough rule of thumb?

Answer 1

In general the advantage of a sharper lens disappears very quickly once you start cropping and enlarging the image.

You can use the lens resolution to work this out: if your lens resolves 100 line pairs per mm and you have to enlarge by a factor of 2 then your effective resolution is 50 line pairs per mm.

These resolution figures are rarely supplied by manufacturers although they often feature on third party lens testing websites - figures from different websites may not be comparable due to differences in testing methodologies.

So the question comes down to whether your 50mm lens resolves ~30% more detail than your zoom. My guess is that with both lenses stopped down it probably doesn't so the zoom would be better.

The best way to decide for sure would be to shoot a comparison yourself!

Answer 2

The facilities that a "high quality slide copier provide are usually, good lighting, maximise contrast by excluding extraneous front side illumination, easy insertion/removal of slide and ease of correct positioning of slide and camera.
In exchange, in the case you are considering, it seems that the slide to camera distance is fixed.

You can make your own copying facility at very little cost, results can be as goo or better that most commercial units and you can set lens to slide distance to optimise whatever result you wish to.

Good illumination would usually be a colour accurate light source with either a condenser light distribution system or, more usually in all but th more expensive systems, a suitably effective diffuser. Emulating the commercial lighting systems may take some playing but should not be hard. Using a bulb and condenser unit from an old enlarger. [ Example only - $20 + postage buys you this condenser on ebay.]

Setting up your camera on a tripod, camera pointing down vertically with slide in a guide above a suitable light source, or camera horizontal screwed into place and slide in a guide with light source behind, should be able to be implemented with minimal cost other than the condenser (if used) and light source.

Then you can use the lens of your choice at the distance of your choosing.

This Gargoyle image search for 'slide copier' provides a wealth of DIY ideas looking at the thumbnails alone - and each is linked to a webpage.

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DIY example - can't complain