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by Bart Arondson

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11

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 ...


10

Distortion caused by a lens's optics would give you barrel distortion (objects appear to bulge outward) or pincushion distortion (squishing inward). The skewed lines you are observing are straight; this is perspective distortion, and is not a problem caused by the lens nor fixable with better optics (you can fix it with a tilt-shift lens, but that's a ...


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 ...


8

This not a thing that can happen. Cameras just don't work that way. For that matter, light doesn't work that way. Specifically, for digital cameras: every "photosite" — each individual pixel-level sensor — is just a counter of photons. It doesn't have any sense of the wavelength of the light received, and correspondingly no perception of color. In order to ...


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 ...


6

You can find a lot of others with this problem, especially some Nikon primes. Apparently, also zooms like yours. It is reflection from the glass in front of the sensor that reflects back to the lens rear element. The colour and power of the spot depends on camera/lens combination, but often it is reddish (pink, magenta, whatever you want to call it) ...


6

You're getting what is called Perspective distortion which is most noticable in wide angle lenses. Check out this link for more info: http://en.wikipedia.org/wiki/Perspective_distortion_(photography) Basically close up objects in the center of the frame will look enlarged while objects on the sides will be stretched away from the center of the photo.


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 ...


6

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 ...


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

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 ...


5

Two reasons: A camera objective (made up from many lenses) needs to focus on a plane, not an arc. And we dont see the image projected on the back of our eye ball. We build up the image from features extracted by many neurons with each their specialty. That's why we don't have to show reality to the eyes for us to see the same, we just need to construct ...


5

The lower the number after the F, the more light gets into the camera. More light means - you can get away with shorter exposure times. - you have lower depth-of-field (which is a good thing in portrait shots) The ranges given for the lenses are the values for the wide angle and the tele setting of the lens. Generally you want the numbers to be as low as ...


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 ...


4

After some reading, this is what I've found: There's no guaranteed indicator of which lenses will show focus shift, but fast primes, especially lenses optimized for smooth out-of-focus blur ('bokeh'), are the most likely to show it. Older designs or those intentionally pursuing a 'classic' appearance are particularly likely. Single aspects of lens ...


4

The SLR Magic Hyper Prime is lower than that at f/0.95, and Leica's Noctilux also offers f/0.95. And then there's the brand new IBELUX 40mm f/0.85. And if rental counts, you can rent the Zeiss f/0.7 lens made for NASA and famously used by Stanley Kubrick - but only attached to a specific camera. That's often claimed to be the largest practically usable ...


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

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, 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 ...


3

This is because of differences in how your eyes and your camera work. In your camera, the rear element of the lens is the final optical element before the image is formed. The aperture iris is placed at or near the point of "maximum out-of-focusness" (the optical center of the lens) so that the shadow of the iris is spread evenly over the entire image. By ...


3

STM and USM are different kinds of focusing motors. Prior to STM, USM was the preferred option because it was fast and quiet (relative to other kinds of motors that are available). However, it isn't fluid which becomes an issue with video. Enter STM which is also quiet (though I'm not sure which is more quiet) and provides a smooth transition as it focuses ...


3

You can look at DxOMark's Lens Ratings, and particularly the Optical Metric Scores, which include a T stop measurement. I don't put too much stock in DxOMark's overall numbers (which don't have much practical impact for real use), but if you're interested in this particular thing, here's a way to tell. Manufacturers do not typically give this number, so the ...


3

Simply put, it does, our brain corrects for it. Among other things, you can't actually see where your optic nerve attaches to the retina and your visual acuity is actually much more center focused than even a cheap camera lens, but because your eyes refocus on the fly every time you change where you are focusing, you don't notice the changes or the lack of ...


3

Here's the short answer: a wide angle lens on a crop sensor skews the image exactly in the way it does in the center of the frame on a full-frame sensor. In turn, this means that using a wide angle lens (small focal length) on a crop sensor gives the same perspective distortion as using a narrower lens (larger focal length) on a full frame sensor, with the ...


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 ...


2

On a zoom lens, it's common for maximum aperture to vary from one end of the zoom range to the other. Typically, the largest aperture (smallest f-stop number) will be available at the wide end of the zoom range, and it will decrease throughout the range until it reaches the smallest maximum value (yes, it's sort of confusing) at the telephoto end of the ...



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