Re: [APML] Question about Loss in resolution

[Ray Butler <ray.butler@nuigalway.ie>]

Michael,

When you say "loss in resolution" and "loss of data", what you are 
really referring to is a loss of perceived sharpness. Fortunately, if 
the photo enlarger or slide projector is correctly focussed, a high 
degree of enlargement cannot cause a loss of either true resolution or 
of data.

The original slide or negative captured an image which has a limit to 
its resolution, imposed by a wide variety of factors: the film grain, 
focus, tracking, vibration, film flatness, lens quality etc. 
Photographers measure resolution in lines per mm (lpm) or more 
accurately line _pairs_ per mm. About 50 lpm is regarded as a good 
resolution figure on the negative/slide. Higher is better, but let's 
take 50 lpm as an example. If you enlarge the negative by 10x to make a 
big print, or project a slide so that its image on the wall is 10x 
bigger, then that 50 lpm becomes 10x smaller = 5 lpm. But there's still 
the same overall amount of detail/data present.

Now the thing that troubles you, this loss of perceived sharpness, is 
all a function of viewing perspective. If you double the enlargement but 
also double the distance you are standing away from it, what you see in 
terms of sharpness and detail will be the same. This is because you are 
equalising the threshold of detail perception. The average human eye 
regards detail of about 7 lpm or greater as sharp for a "normal" 10-inch 
viewing distance, because that's its resolution threshold at 10 inches. 
If you stand too close to the picture, your eyes will resolve more than 
7 lpm on it, but there is no extra detail beyond 5 lpm on the 10x 
enlarged print, so it starts to look soft - what you called the "loss in 
resolution".

Another way of looking at this, is that if you keep constant the angle 
subtended by the picture at your eye, the detail and sharpness always 
look the same. A slide held a few inches in front of your eye and the 
same slide projected onto a 30-foot cinema screen look the same, if the 
angular size at your eye is the same.

And no, there's no difference between slides and prints in this regard.

But in a different way, the luminosity of a projected slide can be more 
satisfying than a print.

Good luck, and keep shooting and enlarging.

Ray "who is reading and posting to APML for the first time in over 2 
months, having been too busy with the arrival of our first baby, a 
gorgeous wee girl" Butler.


Michael Barlow wrote:

>     I have a very nice 4x6 print of the Moon I took this summer, It was so
> nice that I had it blown up to 10x15.  I knew there would be some loss in
> resolution but I had no idea how much so I figured this would give me an
> idea of that loss for other objects should I do this again.
> 
>     I have my first slide film in the camera now and I'm waiting on a good
> night to take some shots of Saturn and M42.  I don't expect to get any shots
> worth the money to have them developed because of inexperience in manual
> guiding, but should I happen to get that perfect shot of something....
> 
>     As mentioned above I now see how much loss of data there is as print
> pictures are blown up in size and I can only assume the same loss ratio with
> slides being projected on the wall.  Is this correct or is the loss ratio
> different with projected slides?  Example: The projected image being 10x15
> on the screen.  Or is there something about slides and/or projecting them
> that helps lower the resolution loss ratio?
> Michael A. Barlow
> 
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-- 
Dr. Ray Butler (ray.butler@nuigalway.ie || ray@physics.nuigalway.ie)
Lecturer, Dept. of Physics || Computational Astrophysics Laboratory
National University of Ireland, Galway, University Road, Galway, Ireland.
Tel: +353-91-524411 ext. 3788   FAX: +353-91-525700

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