Before looking at other riflescopes, I wanted to bring up a point, which I think is important; how can we change the parameters of a given riflescope.
In our current example, we have the focal length at 115mm. We can't change that. We have the objective diameter at 52mm, but that we can change. March supplies a modifier disk with this (and some other) riflescope. This is cap that screws in at the front and it has a hole in it. In this case, the modified disk is 35mm in diameter. This represents a 50% reduction in incoming light, which provides a boost of 1 F-stop to 3.3. So by adding that modifier disk, we increase the F-number. I will also say that a drop of 50% of light is not significant in daylight, unless it's really cloudy and late or early in the day.
Let's see what that does to the hyperfocal distance. We will use f/3.2 because there is no f/3.3 in the calculator. This is a photography calculator and there is no f/3.3 lens.
The hyperfocal distance is now 139 meters and ranges from 69 meters to infinity. What about a near distance DOF? With the focus at 47 meters, we go from 35 meters to 71 meters. So those two distances could be our setup to easily cover from 35 meters to infinity without even looking through the scope.
As we can see the diameter of the objective lens has a big influence on the DOF. Let's keep that in mind when comparing riflescopes. A wider objective will give us more light when that is needed (higher magnification, longer shooting time, and also better resolution when using the same level of glass), whereas a smaller objective will provide a deeper DOF, which some people think makes for a clearer picture, but actually provides for less resolution.This message has been edited. Last edited by: NikonUser,
Let's examine another scope, this time a March-FX 5-40X56. I measured the objective focal length at about 180mm. The objective diameter is 56 so the F-number is 3.1, which is the same as the 4.5-28X52 with the modifier disk.
However, even though the F-number is the same at 3.2, the focal lengths are different, 115 Vs 180. Let's see what that does to the hyperfocal distance.
For the 5-40X56, the HD (hyperfocal distance) is 340 meters, from 170 meters to infinity. That is radically different compared to the 139 meters, from 69 meters to infinity of the 4.5-28X52 with the MD.
Of course, there is also a modifier disk available for the 56mm objective and it does the same as the other one and will add a full f-stop to the number, bringing it to 4.2. Unfortunately the DOFmaster does not have an f/4.2. Only a f/4 and f/4.5. We can calculate for 45, but I'll use f/4.5 for now.
With the MD on that 5-40X56, the hyperfocal distance is now 240 meters from 120 meters to infinity.
Very interesting and informative discussion, NikonUser. Thank you as always.
Let me ask to ensure I understand: The CoC isn’t something that is a measure of lens quality but rather is used to describe image quality; i.e., how far out of perfect focus the image can be and still be acceptably sharp—correct?
You mention sometimes using a CoC of 0.02 or 0.025mm at higher magnifications. Why would it be smaller at higher magnifications?
Added: And I assume that the depth of field calculation/acceptability has nothing to do with parallax—? I.e., to use your one example of sharpest focus at 196 meters and acceptable focus from 98 meters to infinity, that doesn’t mean that if the focus were set at 196 meters that the image would be (necessarily) parallax free from 98 meters on; is that correct?
A pair of good questions, sigfreund.
1- Yes, the CoC is a measure of the quality of the focus for which the lens is capable. I do not profess to know anything about how lenses are assembled, but once I saw the measurements of specific lens placement, my little mind was blown. They were taking about hundredths of millimeters, if you can believe that. So to my mind, the sharpness of the focus is a product of quality glass for sure, but even more importantly, quality of the assembly. This is why it takes so long to build these high-quality riflescopes.
2- An excellent question. I take you back to how a riflescope works: The objective lens group captures the image of the objective and focuses it on the FFP. This is the focus we are talking about here. The quality of the image (IQ) at the FFP is critical for the rest of the riflescope because it will not be improved further down. Now the second part of the riflescope is the erector assembly with the zoom lenses This assembly will flip the image around and will start to magnify it. In fact, that's the mechanism that takes the image provided by the objective lens and then can zoom in 6.22 times. This is like looking at a picture that you took with your digital camera and zooming in on the screen. Any defect in the picture will rapidly show up and the final picture will look crappy. This picture is formed at the SFP. Then you have the magnification provided by the eyepiece itself. It's like a 5X loupe or some such.
So to my mind, when you are using the low to mid range of the zoom, a CoC of .030mm at the FFP is fine, but when you get to 5X or so, you might want to be more critical of the image at the FFP and reduce the CoC for your DOF to keep the IQ of the final image going to your eye.
All right, now I believe I understand what I was asking about. Thanks again for taking the time to explain further.
On a totally different area of optics, Deon has just posted an article at their site that should be of interest for those who shoot at long distances and have to contend with mirage or shimmer.
It has to do with the type of glass used in a riflescope and how ED and especially Super ED glass has been found to have unexpected qualities.
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