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We woke this morning to discover that we’re in the season called Still Winter. 🙂 We don’t usually get snow this late, but it can happen! So I got out my camera to document the beauty of a combined Spring and Winter. Trying to get the nice blurred background I like in flower photos, I borrowed my dad’s 60mm’ f/2.8. I was in aperture priority mode, so I opened the aperture as far as possible. But for some reason, it wouldn’t open all the way to f/2.8. Do I have some setting wrong on my camera, or is my camera doing some kind of internal reasoning that I don’t understand?
These two photos were both with as low an f-stop as possible, but the tulip one was at f/3.0 and the forget-me-not one would only go to f/3.5. So why did my camera (or was in the lens) do that?
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Nice pictures! I like the forget-me-nots; I was just photographing some with an f/2.8 lens the other day as well! But it was a 28mm f/2.8 lens…
Anyway, on to the technical question! My first thought was that zoom lenses are often variable-aperture, like f/3.5-5.6. It varies depending on the focal length that you select. However, the lens in question is a prime, so that’s out of the question! So, I fired up my search engine, and after running for about .5 seconds, it gave me a couple of results that should answer your question! 🙂
Here are the results, in the order that I found them:
First, was a person who had the same question as you.
My Nikon 60mm D Micro or my D800E (with the 60mm attached) have a strange behavior, if i set aperture to f2.8 up to f4.5 when i focus in or out the camera shows different f numbers e.g. f2.8 shows as f.3.5 Sad. This occurs only from 2.8 to 4.5 if i set f5 and above the camera shows the right f number.
And here’s the answer:
You might be confusing the effective and real aperture.
When focussing close, there is light loss due to magnification, and the combination of actual aperture and light loss is known as the effective aperture. Nikon cameras display the effective aperture on the LCD, which confuses a lot of people. (emphasis mine)
– https://www.imaging-resource.com/lenses/nikon/60mm-f2.8g-ed-af-s-micro-nikkor/review/ –
The lens is marked as a constant ƒ/2.8 lens, but in fact the effective aperture will be smaller as you focus at shorter and shorter distances. The following table lists the aperture to distance differences:
Subject distance 6” 7” 8.5” 9” 10” 1′ 3” 3′ infinity
Largest aperture ƒ/4.8 ƒ/4.0 ƒ/3.8 ƒ/3.5 ƒ/3.3 ƒ/3.2 ƒ/3.0 ƒ/2.8
Smallest aperture ƒ/57 ƒ/51 ƒ/45 ƒ/45 ƒ/40 ƒ/36 ƒ/36 ƒ/32So, to answer your question, it is a combination of camera and lens! The lens loses light when focusing up close, so Nikon cameras show a different aperture to compensate so that you can calculate your flash exposure correctly. That’s not very handy for someone who doesn’t use flash! 🙁
Here’s a thread that goes into more details if you’re interested. http://www.dpreview.com/forums/thread/3901125
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Thank you, buddingphotographer! I was too tired when I first saw your response to make heads or tails of it, but now I think it makes sense… tell me if I’m wrong!
So, if I understand correctly, when I’m so close to my subject that the camera’s proximity to the object diminishes light intake, my Nikon shows a different aperture number from what it is really using. And if I stop down as far as I can, my camera is really using f/2.8, but is telling me that it’s using f/3.5 or something else (little liar! 🙂 ) to indicate that it only has as much light as it would at its said aperture focused at infinity. The purpose of this deceitful display ( 🙂 ) is to inspire me to increase my light source in order to compensate for the decrease in light due to my being so close. That said, however, my DOF must still be that of the f/2.8?
You’re welcome @Frazer-Family!
I’m not 100% sure what to tell you, as there seem to be differing ideas on the subject. From what I understand, your Nikon is more truthful than someone else’s Canon or Pentax or Sony.
I’m afraid that it does affect your DOF, just as if you had actually stopped down to f/3.5 or f/4. 🙁 Your Nikon is showing you what your aperture really is, everyone else doesn’t know any better unless they go to the trouble to find out!
Something that most people don’t know about IF (Internal Focusing) lenses, is that to focus so closely (7.3 inches in this case) they actually reduce the focal length. For example, the common Sigma 70-300 “Macro” can focus much closer than a “real” 300mm because as you focus closer, it reduces the focal length to allow you to focus that close! When you change the focal length, you’re necessarily changing the aperture, because aperture is calculated using the focal length.
But just think, if you owned a Canon, you would be happily shooting at “f/2.8” and not even know that your DOF was a miniscule bit larger than you thought it should be! 🙂
P.S. If you think you have it bad, the highly specialized and expensive Canon MP-E 65mm f/2.8 lens has an effective aperture of f/96 when it is shooting at 5:1 magnification!!!!
Well, that’s nice to know that my Nikon is actually being truthful! 🙂 I’ve never been strongly for any particular brand of camera, but I do appreciate honesty. 🙂 So when it says it is doing f/3.5, it really is doing what it says it is. And if I understand correctly this would only be an issue with macro lenses? But I don’t really understand the reason behind it…
When you change the focal length, you’re necessarily changing the aperture, because aperture is calculated using the focal length.
How is that?
And if I understand correctly this would only be an issue with macro lenses? But I don’t really understand the reason behind it…
Yes, it’s only a concern when focusing up really close. At infinity, all lenses are the same,i.e. f/2.8 is always f/2.8
Here’s the formula for calculating aperture:
f/stop number = focal length / aperture diameterSo let’s say you have a 60mm lens at f/2.8. So
f/2.8 = 60mm / 21.42857142857143, or the diameter of the aperture is 21.42857mm. Remember, this is at infinity. Now let’s say you’re focusing at 1:1 magnification, and to be able to focus that close the lens designer had to move the elements around inside, which actually increases the focal length. Here is Nikon’s official statement:The camera’s LCD shows progressively higher f-numbers as an AF Micro-Nikkor lens is focused closer because the optical elements inside are moving farther away from the camera so less light is reaching the film plane. The meter interprets this as a progressively smaller effective aperture, even though the actual aperture is not changing. This is normal with AF Micro-Nikkor lenses and does not indicate a malfunction. (emphasis mine)
Now, if you know a little math, it makes it easy to calculate the reason for the increased aperture. When your camera is showing f/3.5, then the actual focal length must be ~75mm.
f/3.5 = 75mm / 21.42857142857143Remember, your aperture diameter of 21.43… isn’t changing, it’s only the focal length which makes it darker. (The reason that changing the focal length makes it darker is thanks to the inverse square law) Here’s a quote to help explain that law:The explanation is very simple. The lens at 80mm is longer than the lens at 32mm. Light falls in intensity over the distance it travels. So the light at the end of a longer lens is less intense than at the end of a shorter lens. Hence, for a 80 mm lens, if the size of the aperture is larger as compared to the 32mm lens, the amount of light coming in will be the same.
Mathematically :
The inverse sq law states : “The intensity of the light is inversely proportional to the Sq of the distance it travels.”
That is I is proportional to 1/(D * D)
where
D is the distance and
I is the IntensityFor a 32mm lens at f/8
Diameter is 32/8 = 4 mm, therefore radius = 2mm
The area of the aperture is then 2 * 2 * pi = 4pi
The focal length of the lens is 32mm so
the intensity of the light = 4pi/(32 * 32) = 0.01227 ………….. AFor a 80mm lens at f/8
Diameter is 80/8 = 10 mm, therefore radius = 5mm
The area of the aperture is then 5 * 5 * pi = 25pi
The focal length of the lens is 80mm so
the intensity of the light = 25pi/(80 * 80) = 0.01227………….. BWe see that Equation A = Equation B
We can say that the amount of light coming in at f/8 is same across focal lengths.
So, does that answer your question yet? Yet again, I’m learning a lot myself in the researching of these questions, so thanks to you for helping me learn! 🙂
I’m sorry to learn that my Pentax DSLR effectively lies to me when I shoot at 1:1. 🙁 Actually, I don’t own a lens that goes to 1:1 natively, so it doesn’t really make any difference to me!
Wow! This is really great stuff to know! I don’t think I completely understand it, but it sure is fun to contemplate. Having focused on the artistic side of photography so much, I forget about how deep the technical stuff can get. I love it! There’s got to be some analogy there between the Christian life and a “lying” camera. 🙂
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