Revisiting Image Magnification
A short while ago I wrote something about image magnification, film scanning, sharpening etc. The original post was making a point that looking at some of the unsharpened film scans at 100% that I presented while discussing various characteristics of film has nothing to do with image sharpness on the film, in any reasonable sized print, or in comparison with any DX/FX sized camera sensor available.
I also made a point that consumer digital capture has a long way to go and may never get to a place that is easy to get with medium format film. The point that I was making was not anything aesthetic, although I could argue about that as well, the point was all about image magnification and why lower image magnification = better. The way that I explained it caused some confusion and conversation with a couple of readers so I thought I might try to clarify the whole image magnification thing one more time.
Let’s forget about pixels for a second and talk just about the size of the capture medium vs the size of the final output product. If you have an FX or 35mm sized capture device, be it digital or film, and make an 8 inch wide print that corresponds to the short side of your sensor the magnification will be about 8x - end of story. This is because the short side of that capture medium is about 1” by 1.5” Â on the long end (not being exact, just making things simple), hence if you make it 8 inches that will be 8 times as big. Very simple. Let’s say that your capture medium is perfect - your sensor or film has no flaws. Every other flaw in the image making process will be magnified by 8x - movement, vibration, imperfect focus, lens flaws, etc. etc. Yes I know that movement and vibration are affected by focal magnification as well but all else being equal they will all be magnified by that amount. Also realize that there is no such thing as depth of field when talking about perfect focus - there is exactly right focus and varying degrees of out of focus. The more the magnification the more you can see them - yes the “varying degrees of out of focus” also depends on aperture but at the end of the day what is acceptable depth of field is also governed by image magnification.
Here is the part where I confused some people in my original post. I brought pixels and dpi into the conversation. The only thing that pixels and dpi have to do with anything related to image magnification is when you also start talking about viewing at “100%” on your output medium. This is what everybody does to judge image quality, they view all of the pixels at a one to one ratio with their screen - 100%. So let us bring megapixels into our little example - say I have a Nikon D3 12 megapixel camera with an FX sized sensor and I view it at 100%. Assuming you and I have a normal modern monitor we have an output device that is really close to 100 pixels per inch. So we are now going to be viewing 2,832 pixels at 100 pixels per inch or… something 28 inches tall - just a section of it. Using our 3rd grade math skills that would be an image magnification of, hmmmm 28x.
Are we good so far? That 28x is like really, really microscope-ish. To tell you the truth things really start to fall apart at a close viewing distance at maybe 10x or 15x - if you are far away they will still be fine but close up they fall apart. Now let’s say that we keep the same lenses, and more importantly the same shooting technique and conditions. Same flash units whatever but we buy ourselves a brand spanking new Nikon D3x with a small size pixel dimension of Â 4,032. Woo hoooo our stuff is going to be so much better - wrong. It will probably be about the same but now when we go look at it at 100% we get to see it at 40x magnification - gee golly. Now we are well into microscope territory and I can bet you your focus sucks, autofocus sucks, your lenses suck - oh and your shooting technique and conditions really really suck at 40x.
When you are getting more megapixels you are getting the ability to magnify more with out your pixels turning into squares but you are doing nothing about the magnification factor that will always be a killer depending on your output intent and viewing distances. Hey if you are a hundred yards away you can output billboard sized stuff and it will look fantastic. Now if we get back into practical sized stuff that we deal with close up - how big do you want to go and how good do you want it to look. Just about anything looks great at 4x and below, I really do mean just about anything. Reasonably competent stuff looks good up to about 8x. I am talking about really close up.
Beyond 8x and technique, lenses, etc, etc become much more critical. For instance I was shooting some fashion stuff a long time ago with my lighting kit that is perfectly fine and plenty powerful. The problem was I was shooting on medium format for something that involved a lot of action and the target display size was going to be about 20x. Guess what - my flashes had a duration of 1/750 second - pretty fast but at 20x you could see motion ghosting in the models hair. Not good for what the job was. I had to spend a ton of money on some profoto packs and heads just to get a flash duration of 1/2000s to make the ghosting go away. The images would have been perfectly fine at 10x. If I could have I would have shot it on 4x5 but there was no way to do that and get the job done in any reasonable amount of time - if ever. I definitely needed to shoot more than a frame or two to nail the shot.
Fast forward to my posts and medium format film - when I scan medium format film that has a short side of 9000 pixels and show you an unsharpened “100%” view you are seeing a piece of something that is 90 inches wide from a foot away or… 40x magnification. Ta….Da…. Something that most of you have never seen - a 40x magnification of something under a microscope that was hand held shot in available light at 1/125 second at f4 and quickly manually focused on 400 speed black and white film. In the real world of stuff we might make with that negative like a big book it will not only look fantastic but quite possibly look better than anything shot on a smaller format no matter what the pixel count because of the fact for the real world stuff the magnification will be lower due to the larger capture device.
I hope I did not confuse the matter even more. Bottom line - when making pictures image magnification counts a lot - the lower the better - not just pixel count. In fact unless you are actually running into issues where you need to make images bigger and do not have enough pixels to do so than they don’t do much. As for cropping - sure go ahead but not too much - no matter how many pixels you have to crop out of, you are increasing image magnification. If you think you are going to crop out a head from a crowd and it will be a fantastically beautiful headshot because you have more pixels you are dreaming. You really do need to get the image size “about” right on the film/sensor/etc. if you do not want it to fall apart. Hence - If you want to occasionally make really big images that look great you may find it easier and cheaper to grab some of the fantastic medium format film gear that is out there.
Ps. EVERYTHING at 40x microscope magnification looks like crap, all movement and vibration starts to show up, unless you focused at 40x magnification your focus looks like crap, EVERYTHING looks like it sucks. When will you make output that is 20x magnification designed for close viewing? NEVER - if you have to you are using the wrong device to capture your images, get something bigger and go for lower magnification for the same target output size.blog comments powered by Disqus