What Rio Rico said. Resolution limits of what the eye can perceive are restricted by the distance between the cones at the back of the eye. Anything which has a circle of confusion smaller than a couple of cones wont be seen by the eye. The resolution limit of the eye depends on whether the image is a dot or a line. The eye can see a line more easily, as it crosses many cones. In John Williams' book called Image Clarity, there was an argument for the eye resolving 30lpmm in a photo at normal viewing distances (hand held photo). It depends on the shape of the image...breaks in a line can be perceived more readily than points.
Less stringent requirements would be 10lpmm quoted in Quality in Photography by Hicks and Schultz. 10lpmm equates to 20 pixels per mm...so the 4500x3000 pixels could be printed on paper 225x150 mm or 9x6 inches, but it depends on the 25cm viewing distance....if you stand twice as far away, you get 20inch viewing distance and 18x12 inch print, a bit more than Rio Rico's estimation. This is all a bit handwavey, and depends heavily on your eyesight. But this is a massive simplification...
You must do a lot correctly to get the full resolution out of the sensor, such as using a tripod or shutter speed in excess of 1/500s to overcome mirror slap/camera shake/object motion. You obviously need a lens which has a higher resolution than the sensor, for an acceptable MTF value, say 50%, with a decent hood on it, and you need a well lit subject with sensible tonal range which offers the requisite detail (not too contrasty, but with good detail). Indeed, you can have a high resolution image with poor contrast and gradation in it. Information in an image =[(resolution) x (range of tones)]....this is where the importance of the subject, lighting, tripod, lens and hood come in. How you post process probably has a large effect on it, with RAW processing requiring the same consideration as developer selection in the old days...high accutance developers are analogous to sharpening, emphasising low resolution detail at the expense of higher frequency information. So you probably want to turn your sharpening down. 50% MTF value at the level of resolution the sensors offer is pretty impossible. The sensors see 120pixels per mm or 60lpmm...I doubt there are many lenses which can achieve this. Hence the need for larger formats.
I've taken a 35mm photo on FP4+ which had a bill-poster advert in the background, about 10cm tall, on a 3m section of wall, showing a phone number on it. The poster is therefore about 1mm on the negative, so the numbering is about 0.1mm on the negative yet you can read the number. I reckon I might have got close to 50lpmm on the negative, with a sufficiently high level of contrast to read something. I was using a Pentax M 100mm 2.8, with the Tak 135 hood, and had a shutter speed around 1/500, using a ME Super (hard to tell what the shutter exactly is with the ME Super). I've only printed it to 5x7, which means the writing is only 0.5mm high, however, there is something neat about taking a loupe to a back lit 5x7 photo and seeing detail in it which couldn't be seen by holding it in the hand. Makes it feel like a world is trapped inside the photo. At some point, I will pony up the cash for some bigger paper and try hanging it on the wall. But this raises the point, for me at least, that its best to print within the resolution limit of the photo, not at it, as it means there is further detail accessible when you put your nose right up to it, sort of a gift for really studying the photo...makes it worth while printing it out, rather than leaving it on a hard drive!
[here is an added conundrum...how close can you get to a print before your eye can outresolve the detail the paper can record? I've just had a look around and it sounds like 600ppi for modern paper which is about 12lp/mm which means no closer than 15cm....this probably corresponds to a normal person's minimum focussing distance.]
Last edited by whojammyflip; 04-03-2012 at 07:45 AM.
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