The question of how lens focal length and binocular magnification appears to have been asked over 3.86 quinzillion times. Regardless, it's still hard to get an answer the layman can understand. I'd like to have another go by asking a very specific version instead of the more general question.
- I'm into back-yard birding, for which I currently use a pair of 8x magnification binoculars (Nikon Monarch M511 8x42 6.3° to be precise), and I now want to add bird photography to my activities
- I already have a camera in the form of a Nikon D5200. I believe its sensor has a "crop factor" of 1.53 (and I am led to believe that number is important)
- So, I'd like to obtain a lens that I can use with my D5200 so I am able to see a bird in at least as much detail (i.e. magnified at least as much?) as I do when I look at it with my binoculars, specifically in the following two situations:
- When I look at it through the viewfinder of the camera
- When I look at it in the end photograph
Some other (possibly) relevant info:
- My primary interest for now is observing the bird and recording its image; i.e. it is not in producing aesthetically pleasing bird-ish art. And so specifically, I don't care about depth of field -- or, rather, I don't care if I cannot achieve a shallow DoF. (Do I?)
- I live in a region with very bright sunny days, and most of the birds I'm concerned with are stuffing their fat asses with food from my feeders, so they're not exactly in a hurry. In other words, the speed/aperture of the lens isn't that important. (Is it?)
- Of the two aspects of my lens requirements -- looking through the camera viewfinder, vs looking at the end photograph -- the first is more important to me (because when it's live, through the viewfinder, it might fly away, so the more detail the better; once I've taken the shot, less detail is compensated for by now-unlimited time in which to study it)
So, given all of that, my main question is simply:
Q1. Which focal length of lens do I need?
If I could get a straight and simple answer, ideally in the form of a number of millimeters, that would be WONDERFUL. Provide it and you may consider yourself a candidate for the next Nobel Prize in Straight and Simple Answers on photo.stackexchange.com (it's going to be huge!), or at very least a Truly Wonderful Being.
HOWEVER, based on reading the countless other attempts to ask this kind of question, and their answers (OK, I admit I may only have read 10 to 20, but there are a lot), I accept that in the underlying optical equations there are umpteen other variables over and above just focal length, so in case it's simply impossible to answer Q1 based only on what I've told you so far, here is a followup:
Q2. If my description does not provide enough information to answer Q1, is it possible to specify (and if so, could you so specify) what else you'd need to know to enable an answer?
For example, does it matter how far the birds typically are from where I observe them? Does it matter if I first chuck a brick at all the boring old White-winged Doves so as to clear the way for the spectacular Painted Bunting I actually want to see?
Finally, a pragmatic closing question. I'm pretty sure the number I'm looking for is something in or around the 300mm to 400mm range and for me that might represent a bit of a price pain point: 300mm or less is OK; 400mm or above is a bit ouchy. Therefore:
Q3. If the answer to Q1 is something above 300mm (e.g. 428.4mm if I had to take a rough guess), and I decided I just couldn't stretch that far, just how inferior to my 8x42 binoculars would that 300mm lens's image be, especially in the all-imporant how-it-looks-live-through-the-viewfinder department?
P.S. In case there are any Brit birders reading this, especially fans of Bill Oddie, yes, yes I know that for identification purposes it's better to learn to draw the wee buggers rather than to take pics of them, but I draw the way Hoopoes congregate in Trafalgar Square, so I want a lens!