Choosing what telephoto lens to buy for your needs is one of the most difficult tasks as a photography enthusiast. Apart from the budget and image quality considerations, a big challenge is balancing reach vs. portability vs. speed. Pick any two. Modern camera technology now allows us to simplify this conundrum somewhat.
The Need for Speed
Before we go further into this article, let me just pause here to say that if your objective is to shoot still or slow subjects using a tripod, you could skip reading this and just get the lens that gives you the reach you need at the size/weight limit you can bear. The cost mostly, generally, follows the size and weight. Bigger, heavier glass is costlier.
If, on the other hand, your goal is to shoot moving subjects hand-held, things get complicated very quickly. You might want to use your telephoto lenses to shoot motorsport, people sports, big birds in flight, small birds in flight or airshows. That is essentially, also the order in which your need for shutter speed increases.
Now let’s do a bit of math, shall we? According to the Sunny 16 Rule, you’ll be at 1/64s when shooting at base ISO in broad daylight if your aperture is ƒ/16.
This is too slow, even for hand-held panning motorsport shots, which is the slowest kind of telephoto action photography. Even in motorsport, for Formula 1, you’ll need to go faster. Switching to people sports you might want to go up to 1/1000s, which is also the minimum starting point for slow birds in flight. 1/1000s requires ƒ/4 aperture in broad daylight at base ISO.
44 | Lewis Hamilton
All this is to say that when you are doing action photography – especially bird photography that happens during darker hours – you will be shooting higher than base ISO. In other words, for action photography:
Axiom 1: You will be limited by the light-gathering ability of your lenses.
Factoring the Crop
One of the things that everyone really goes for while evaluating lenses is sharpness, so let’s talk about resolution.
Digital photography is now already 25+ years old, so I’ll not talk about film, except to say that I haven’t seen 35mm film scans that exceed ~ 6MP resolution. This is not to say they’re low quality. They simply don’t have the detail when considering lens performance, graining, etc.
6MP is still quite a lot of resolution. It allows going past 50cm (20 inches) wide in a print at 300dpi. At 3:2 aspect ratio, it almost fills a 4K screen in height (2000px tall vs. 2160). I would happily settle for 4MP of high quality stuff. So that brings us to:
Axiom 2: You do not really need more than 6 Megapixels
1350mm, 3x Crop (4.5x FX), 3.84MP
Many years ago, I wrote a post on how we can leverage this threshold to optimise for pixel quality rather than quantity while leveraging the ability to crop from high resolution sensors.
The crux of the argument is that instead of buying a zoom lens for a crop sensor camera, you should rather buy a wider (therefore smaller) prime lens for a large sensor camera so that you can achieve much higher quality at shorter focal lengths and get nearly equivalent quality at longer focal lengths.
The Scoring Formula
If you look at the specification of a lens, it goes like XXXmm ƒ/Y.Z. The maximum aperture is specified as a fraction of the focal length. Which means, dividing the two numbers (XXX ÷ Y.Z) gives us the diameter of the len’s aperture when it is fully open. That’s basically the “hole” inside the lens that lets the light through. The larger the aperture, the more light the lens gathers.
So from Axioms 1 and 2, given that we are always light limited and we can crop quite a bit, a telephoto’s potential is proportional to its light gathering ability. This ability is measured by the aperture diameter, which is what we use as the score. Here’s a table of some common telephoto specifications and their scores:
| Lens Specification | Calculation | Score |
|---|---|---|
| 800mm ƒ/5.6 | 800 ÷ 5.6 = | 143 |
| 400mm ƒ/2.8 | 400 ÷ 2.8 = | 143 |
| 800mm ƒ/6.3 | 800 ÷ 6.3 = | 127 |
| 300mm ƒ/2.8 | 300 ÷ 2.8 = | 107 |
| 200mm ƒ/2 | 200 ÷ 2 = | 100 |
| 600mm ƒ/6.3 | 600 ÷ 6.3 = | 95 |
| 500mm ƒ/5.6 | 500 ÷ 5.6 = | 89 |
| 400mm ƒ/4.5 | 400 ÷ 4.5 = | 89 |
| 300mm ƒ/4 | 300 ÷ 4 = | 75 |
| 135mm ƒ/1.8 | 135 ÷ 1.8 = | 75 |
| 800mm ƒ/11 | 800 ÷ 11 = | 73 |
| 400mm ƒ/5.6 | 400 ÷ 5.6 = | 71 |
| 200mm ƒ/2.8 | 200 ÷ 2.8 = | 71 |
| 85mm ƒ/1.2 | 85 ÷ 1.2 = | 71 |
| 600mm ƒ/11 | 600 ÷ 11 = | 55 |
| 220mm ƒ/4 | 220 ÷ 4 = | 55 |
| 400mm ƒ/8 | 400 ÷ 8 = | 50 |
| 200mm ƒ/4 | 200 ÷ 4 = | 50 |
| 105mm ƒ/2.8 | 105 ÷ 2.8 = | 38 |
| 200mm ƒ/6.3 | 200 ÷ 6.3 = | 32 |
| 120mm ƒ/4 | 120 ÷ 4 = | 30 |
How to Use the Scores
This table throws up some interesting results. Near the top, you can see that the 800mm ƒ/5.6 and 400mm ƒ/2.8 lenses have the same score. This means that if you are light limited, you can get similar quality of image after cropping from a 400mm ƒ/2.8 lens as you would from an 800mm ƒ/5.6 lens. In fact, the 400 ƒ/2.8 will continue to auto-focus further into twilight than the 800 ƒ/5.6. Using Nikon lenses as examples, the Z 400mm ƒ/2.8 lens is also cheaper and much more portable than its uncle, the AF-S 800mm ƒ/5.6E FL VR Nikkor.
Opposites Attract
There is also something to be said about the 4x faster shutter speed that the 400mm ƒ/2.8 will give you over the 800mm ƒ/5.6 at the same ISO. It helps with shots like the one above. With slower shutter speeds, one of those planes would have been blurry.
Of course, if you are not light limited, or you need to crop even further from 800mm (e.g. maybe you want to shoot the moon), the 800mm ƒ/5.6 lens will be the better choice.
Half Moon, 3000mm
While comparing lenses here, you must remember to account for how much cropping you can get away with. I am quite comfortable with up to 5MP crop, which gives 3x zoom on a 45.4MP Nikon Z8 body. Let’s explore this with an example.
Looking at the scores, the 135mm ƒ/1.8 is the same as the 300mm ƒ/4. The crop from 135mm to 300mm is 2.2x, which is well within my comfort zone. Yet, I have both lenses, a Z 135mm ƒ/1.8 Plena and an AF-S 300mm ƒ/4 PF VR. With the FTZ adapter on the latter, they are both similar in size and weight too. Why the duplicity? Actually, I use the 300 more as a 300-900mm zoom, while the 135 is for general travel telephoto and motorsport. The airplane shot above was taken with the 300mm ƒ/4 cropped down to a cool 1350mm equivalent.
400mm, 3x crop, 5MP
The above photo is proof that the scoring works. I occasionally shoot bats in flight and it’s so dark when they come out that I’ve never managed to get a good shot with the 300mm ƒ/4. In the link above, you may see shots from a 500mm ƒ/5.6 PF VR, a 70-200 ƒ/4 and even a Z MC 105mm ƒ/2.8. The shots tend to follow the scoring.
Further down the table, you can see that the 200mm ƒ/6.3 and 120mm ƒ/4 lenses have the same score. No wonder then it’s so difficult to choose between the Z 24-120mm ƒ/4 S and the Z 24-200mm ƒ/6.3. All I can say is, they are both similar in telephoto potential! By the way, if you are curious about the 105mm ƒ/2.8, outscoring the 200mm ƒ/6.3, I’ve got something to show you:
600mm, 5.7x crop, 1.4MP (don’t mind the haze)
Caveats
While this scoring gives the potential of various lens specifications in light-limited situations, it assumes perfect lenses, infinite resolution sensors and no diffraction. In reality, the validity of your results will depend on the quality of specific lenses and sensors involved. Roughly speaking, for the same sensor characteristics, the scores will be valid for lenses of the same generation and optical quality.
Also, do understand that very large or very narrow apertures will tend to skew the results. For example, the 800mm ƒ/11 lens is not a good choice for action photography, even though it scores higher than the 400mm ƒ/5.6 because continuous AF systems barely work at ƒ/11 and diffraction softness becomes easily apparent. Likewise, the 85mm ƒ/1.2 lens can not be compared to the 400mm ƒ/5.6. There is no overlap in their use cases despite the scores being identical.
Conclusion
Comparing telephoto lenses using the aperture diameter as a score for their light gathering potential is an effective way to understand the trade-offs between reach and portability. With the advent of high resolution sensors, we get the “Prime as a Zoom” capability that can be stretched reasonably far. As always, the suitability of a lens depends closely on the specific situation, but this scoring system can help in narrowing down your target specifications when looking for a new lens.