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by Gordon

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13

Many older or cheaper phone cameras use a "fixed focus" lens. ie it is always set to focus a specific distance away from the camera. This is usually set to the "hyperfocal distance", ie everything from half that distance out to infinity is in focus. This depends on just what is acceptable as 'in focus'. But most photos from these cameras will be sharp ...


12

The main reason we don't have "super zooms" with a large constant aperture is size/weight/costs. Roger at LensRentals recently blogged about this in the post: About That 25-300mm f/2.8 You Wanted About How Big is that? The lens is in a video housing, so that makes it a bit larger than an SLR designed lens of the same specifications would be. But ...


11

There are two hard limits on how fast a lens can be: The first is a thermodynamic limit. If you could make a lens arbitrarily fast, then you could point it to the sun and use it to heat your sensor (not a good idea). If you then get your sensor hotter than the surface of the Sun, you are violating the second law of thermodynamics. This sets a hard limit at ...


9

All lenses create a circular image, it's just that most of them have an image circle large enough that it covers the entire sensor. Vignetting at wide apertures is a manifestation of the image circle encroaching on the corners of the sensor as the circle edge is not as sharp as it would be with a narrower aperture. With a fish-eye lens, the image circle is ...


7

It doesn't look too bad to me. You have to consider that when you're looking at a 5D mkIII image at 100%, that amounts to a considerable enlargement. It's rare to get something really pin sharp at that magnification. The focus point is quite forward so the trees in the centre of the frame are at or near the far limit of the DOF. That combined with the ...


7

It is more about ratios than addition/subtraction. 70-200mm is less than 3x from the shortest to longest focal length. 18-135mm is 7.5x. The higher the ratio between the shortest and longest focal length, the greater the difference between the "effective aperture", more properly called entrance pupil, for the shortest and longest length at the constant ...


7

I think that you are underestimating how bad a bad lens truly is. It is going to be very obvious if you pickup a real dud and it is acting like a $500 vs $1,600 lens. You can and should run though the related question and its recommendations for testing any new lens: How can I test a new lens to make sure it is operating correctly?. You state that you ...


6

Bokeh is formed by many points of light spreading out, passing through the aperture and being projected onto the image plane as series of overlapping discs (assuming a round aperture). This can lead to harsh textures and effects when there are strong contrasts in the out of focus parts of an image, especially when lenses feature overcorrected spherical ...


6

The shape of the bokeh is related to the apparent shape of the aperture of the lens. Straight on, this will produce a bokeh that is approximately a circle. As the subject moves away from the center of the field, the bokeh starts to look like a sliver of the circle. This can be reduced by stoping down the lens. (above image from ...


6

The "telecompressor" you mentioned is a focal reducer, a device that concentrates the image in order to project it onto a smaller sensor. This approach reduces the backfocus distance (the distance from the back of the lens to the sensor). So it only works on mirrorless cameras using lenses designed for DSLRs (which have sufficient backfocus distance to make ...


5

This is not "obviously irrelevant to normal photography" at all; we just don't normally worry about the sort of precision that you'll need to deal with. There are two numbers that we ordinarily take at face value, knowing that they're slight fibs: the focal length of the lens (which is usualy rounded to a "friendly" value except on data sheets), and the ...


5

Your logic is sound. If your assumptions were right, then your conclusion would be right. Let me turn one of your questions around. You ask: Why does crop factor apply with APS-C-lenses, while it sounds like the image circle is compressed onto the APS-C-sensor (thus making a wider FOV)? In fact, the image circle isn't compressed, and does not make a ...


5

The blur can be measured by converting to XYZ colorspace and zooming into a tree trunk with a bright sky as the background. You then measure the brightness profile accross the rapid change in brightness (make sure you pick an area with small gradient in the direction parallel to the tree trunk). I then used this method to estimate the blur. Since the image ...


5

According to a related discussion on dpreview: A lens can be made physically shorter than its focal length by the use of additional lens elements called a telephoto group. According to the Wikipedia page for Telephoto lens: The basic construction of a telephoto lens consists of front lens elements that, as a group, have a positive focus. The focal ...


5

You are effectively asking about the geometric behavior of light...rays extending from points, passing through lenses, being bent, and focusing somewhere. This is a very well understood model of the behavior of light, and there are some excellent resources out there that cover the topic. It is much too involved to cover here, so I'll just quote from a couple ...


4

Yes, you can buy non-stabilized optics for a Pentax. In fact, very few stabilized lenses are available, because all Pentax digital bodies since the 2006 model K100D (excluding K110D) have provided the Shake Reduction sensor stabilization, so there's no need to have stabilization in lenses. Yes, image quality is comparable to optic stabilization since it is ...


4

Consider this review of the Canon 8-15 f/4L USM fisheye, which can shift from circular at 8mm to diagonal at 15mm. Yes, it's circular because the lens's entire image circle is inside the area of the sensor, rather than covering the entire sensor. I'm not sure there's going to be an entire explanatory webpage other than Wikipedia on this, because it's such a ...


4

Yes on normal lenses the area in focus is nearly perfectly described as a plane and the small deviations are rarely taken into account in regular photography. There are two important factors that cause these slight imperfections: the Petzval curvature and Astigmatism. The uncorrected astigmatism is usually more severe but can be over-corrected in order to ...


4

Because most smartphones without an additional external macro lens do not have optical systems capable of taking photos at close enough minimum focal distances to produce macro photographs. Even if you define macro as the equivalent magnification needed for the tiny sensors found in most phones to produce a print comparable to one made when the image of a ...


4

These lenses work basically like a magnifying glass (or reading glasses) in front of the lens. In fact, not just basically — actually exactly. That has the effect of decreasing the distance from the back of the lens to where an in-focus image is formed, which gives the lens more freedom to focus more closely. And focusing more closely is inherently all ...


4

They are referring to the amount of clearance between the lens rear element and the sensor. The C mount flange focal distance is 17.52mm so both of these lenses have rear elements that stick into the camera body. This fact is most relevant to SLR cameras that have a mirror which moves out of the way when a photo is taken, a lens that sticks into the camera ...


3

This is field curvature. (And a nice example of it!) Simple lenses naturally project a curved field, not a flat one to match film or digital sensors. Modern lenses attempt to correct for this, but many older designs do not. In fact, it's sometimes called the "Petzval effect" after a classic design famous for this look. Interestingly, just this week Sony ...


3

PF and DO terminologies are nominally interchangeable; Canon holds patents for Diffractive Optics lenses: http://www.cameraegg.org/new-canon-do-patents-500mm-f4-500mm-f5-6-600mm-f4-800mm-f5-6/ .. while Nikon has "Phase Fresnel" patents: http://nikonrumors.com/2015/01/06/nikons-phase-fresnel-pf-lens-explained.aspx/ I believe that their technologies are ...


3

The fisheye "effect" is dependent only on the angle between the camera and subject, it is thus totally independent of distance. What you might be noticing is that a fisheye lens bends all straight lines unless they pass through the exact centre of the image. In some natural scenes the horizon will be the only straight line in the image, thus if you happen ...


2

Just to give another (maybe wrong) view on this, I think what is being referred to as the Leica look is a mixture of subjective and objective things. There are some photographers that are so used to review, make, create, and study pictures taken with Leica lenses and cameras that can identify the lens a picture was taken with. I know it may seem disturbing, ...


2

Why do light sources appear as stars sometimes? Well, I changed my opinion and share now the prevalent one that the stars come from diffraction effects. The strongest argument for favoring diffraction over reflection comes from the symmetry properties of the star pattern, namely, if N is odd, then N iris blades generate 2*N spikes.


2

The mathematics and geometry your working with apply to SIMPLE LENSES. A single lens element with a simple convex curvature of negligible thickness. They also technically only apply at hyperfocal distance in air (vs. water, or vacuum, etc.) The Wikpedia states this: As mentioned above, a positive or converging lens in air will focus a collimated beam ...


2

You're right that the focal lengths aren't that different. Considering that alone, I wouldn't worry about it, unless you have a particular need. (35mm is a little nicer close quarters, but not by much.) But note that that's not the only difference — the lenses have different optical construction, with the 35mm having an additional element. They do seem ...


2

I can't look at the RAW image at the moment, but it should be fairly sharp, but likely won't be pixel sharp on a 5D Mark iii at 100% magnification. The 24-70 f/4L isn't a prime lens and it isn't the f/2.8II. It isn't as strong of an optical performer and 22mpix is a lot of image data. (I use the f/2.8 II on my 5D Mark iii regularly.) The f/2.8II will come ...


2

If you have browsed through the technical pages of Pierre Toscani, you probably noticed he is quite knowledgeable when it comes to geometrical optics. Although I cannot ascertain his schematics are correct, I certainly trust him on this, as this is an extremely well researched article. Concerning the maximum possible lens aperture, Toscani says that since ...



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