question about F8

[kevin_b.2]

<p>Is F8 brighter and or sharper on a 1.2 max aperature lense compared to say a 1.4 or 1.8? </p>

[andylynn]

<p>"Brighter", no - F8 allows much less light to pass. When doing the exposure, this is compensated for by using a slower shutter speed.</p>

<p>Sharper, probably. Most lenses are sharper when stopped down a few notches than they are wide open. You will also have more depth of field - more things will be in focus.</p>

<p>However, an f/1.2 lens is an expensive lens, it ought to be used at large apertures for compressing the depth of field and getting a fast shutter speed.</p>

[lex_jenkins]

<p>In terms of exposure, f/8 is f/8, regardless of the lens or maximum aperture of a lens.</p>

<p>For example, if the correct exposure for a daylight photo at ISO 100 is 1/500th of a second at f/8 for a lens with a maximum aperture of f/1.2, it would also be appropriate for a lens with a maximum aperture of f/1.4, f/1.8, f/2.8 or even with a slow telephoto having a maximum aperture of f/8.</p>

<p>In terms of sharpness it depends on the lens. With a typical fast lens used in 35mm, medium format and most dSLRs, f/8 would be considered a middle aperture and would be relatively sharp. On a point and shoot type digital camera or cell phone camera, with a tiny sensor, f/8 would be a very small aperture and might be approaching diffraction and softness. With a slow, variable aperture lens with a maximum aperture of f/5.6 or f/6.3, or any of the inexpensive 500mm f/8 telephoto lenses, f/8 would not be stopped down much, if at all, and would probably not be as sharp on such a lens.</p>

<p>And with large format film cameras, f/8 would not be a very small aperture and would not be as sharp, relatively speaking, as a smaller aperture of around f/32 to f/64.</p>

[Matt Laur]

<p>Of course, a faster lens <em>does</em> make for a brighter viewfinder while you're composing (since the lens doesn't stop down until the actual exposure), and it also provides more light to the focusing system. To the extent that getting the focus point better established just where you want it contributes to a final image that's sharp in the right places, then the faster lens does help. But that's more of an issue in lower-light shooting.</p>

[frank_skomial]

<p>"Is F8 brighter ... on a 1.2 max aperature lense compared to say a 1.4 or 1.8? " - not brighter. If properly exposed, both pictures will be equally bright.</p>

<p>"Is F8...sharper on a 1.2 max aperature lense compared to say a 1.4 or 1.8? " - Yes, most lenses are sharper at F8 when compared to F 1.2.</p>

[matthijs]

<p>Though I hate to disagree F8 is not always the same amount of light. Using F8 on my 500mm mirror lens gives me way less light through the lens than F8 on my better lenses.</p>

<p>But in general; F8 on any lens of a certain quality equals to the same amount of light on the sensor in comparable conditions. The mentioned 50's are all of sufficient quality to make this true in this case.</p>

<p>I'd hazard a guess that the differences in sharpness between a 1.2, a 1.4 and a 1.8 50mm lens all shot at F8 are invisible.</p>

[michaelkh]

<p>Frank, Kevin asked, I think, if f/8 is sharper on an f/1.2 lens than on an f/1.4 or f/1.8, not whether f/8 is sharper than f/1.2. Matthijs is probably right that the difference is insignificant.</p>

[john_lynch5]

"Though I hate to disagree F8 is not always the same amount of light. Using F8 on my 500mm mirror lens gives me way less light through the lens than F8 on my better lenses."

 

This is wrong. F8 is F8 and is always the same amount of light at the time of exposure. Looking through the viewfinder, the "better" lens will be brighter because the lens is wide open, not at F8. The mirror lens has a fixed aperture of F8. If you set the "better" lens at F8 and close down using the DOF preview, the brightness will be virtually the same.

[lex_jenkins]

<p>Matthijs, you're seeing the differences between nominal specifications and actual performance. It's not at all unusual for actual, measured performance to differ from nominal specifications with any aspect of photography.</p>

<p>That's why the best equipment reviews also test actual ISO, actual white balance results, shutter speeds, apertures, etc. It's not at all unusual for any lens or camera shutter speed to vary 1/3 EV, plus or minus, from nominal specs. This much deviation usually provides acceptable results. Beyond a 1/3 EV deviation, we have to make accommodations to our exposures.</p>

[dave_s]

<p>Matthijs and Lex are both correct. Relative aperture is the most important determinant of illumination at the film plane, but the correlation is not exact because of light loss inside the lens. </p>

<p>The best example of this is in lenses with uncoated surfaces, which may interfere with light transmission due to internal reflection. Some cine lenses express light transmission as a 'T' value, like T8, in order to correct for this difference. </p>

[matthijs]

<p>I know it was nitpicking but the difference is there.</p>

<p>Especially with my crummy mirrorlens.<br>

1. The quality of the glass,<br>

2. the back of the mirror that's mounted in reverse in the front element stops light and<br>

3. the quality of both mirrors all add up to a significant amount of lost light.</p>

<p>(There's also a persistant rumor that the 50/1.4 is about a third stop brighter than other lenses but I haven't tested that. Maybe it's just a story, maybe it's because of the quality of the glass and the limited amount of glass between the sensor and the subject...)</p>

[kelly_flanigan1]

<p>An F8 500mm mirror lens is often dimmer than a 400mm non mirror lens set to F8; the actual T stop of the 500mm lens is often about say T10; darker.</p>

<p>Older single coated (non mulitcoated) complex zooms are often "darker" at F8 than a simpler 5 element short prime tele set to F8 ; like a 135mm F2.8; the old zoom might T stop at 1/3 to 2/3 stop less light.<br>

<br /> A T stop is used for exposure; an F stop is used for DOF tables and exposure too.<br>

<br /> A given F stop can be darker with a complex lens than a simpler one due to light loss with many elements; or the main center hole in the objective mirror of a 500mm mirror "lens".<br>

<br /> A lens that is T stopped means one finds the actual transmission light transfer function the lens/mirror; and gives the lens a Tstop rating for exposure; ie a F8 mirror lens might be T10.</p>

[cathy_and_david_bock]

<p>If you're shooting at F8 don't waste money on fast L glass, it's silly. The point of having that fast glass is to shoot wide open. :) </p>

[lex_jenkins]

<p>There are many advantages to fast lenses other than using them wide open. Faster lenses offer a brighter view for composing and help autofocus to work better in dim lighting. A faster lens is also more useful in combination with a teleconverter or extension tube, both of which can make a slower lens unusably slow.</p>

<p>Faster lenses are also often among the better made lenses from any manufacturer, with fewer compromises in components and build quality.</p>

[cathy_and_david_bock]

<p>Much agreed. Well said. </p>

[vic_leng]

<p>if i'm not mistaken.. they say 2-3 stops down would be the sharpest from the lens's max AV..</p>

[William Michael]

<p >In the context of addressing the<em> question asked</em> (i.e. an <em>F/1.2</em> Lens compared with an <em>F/1.4 lens and an F/1.8 lens</em>) the issues of brighter viewfinders; AF speed; image quality . . . etc will be of little consequence. </p>

<p > </p>

<p >Two practical examples of lens comparisons, which fit the criteria of the question: there will be little difference between the nikon or canon 50mm or 85mm Prime Lenses, if they were set at F8, IMO.</p>

<p > </p>

<p >However, in the broader comparisons of lenses, all these issues mentioned above will have more impact and that is why many photographers will choose a fast lens, even if they do not use it wide open often, or at all. </p>

<p > </p>

<p >As one example, comparing a 35mmF/1.4 and a kit zoom lens (17mm to 55mm F/3.5 to F/5.6 set at 35mm max Av = F/5). When both of these lenses are worked at F8, I expect there would be a truck load of differences.</p>

<p > </p>

<p >I mention this to quantify / allocate the differences and impacts being discussed.</p>

<p ></p>

<p >***</p>

<p > </p>

<p >FYI: T Stops are used in cine work, often / always. Cine lenses are usually complex, (as mentioned) and exposure is critical, with most cine film stock.</p>

<p > </p>

<p >WW</p>

 

[profhlynnjones]

<p>Hey guys, stick with Lex, he is certainly right in his answers here.</p>

<p>Matthijs- There is more to it than you are perceiving (I'll stick my nose in, I was VP of Celestron and dealt with some of these mirror/lens sitluations). Catoptric: mirror imaging system. Dioptric: lens system (light transmitted and altered by the lenses). Catadioptric: the combination of the two types of imaging as in a Newtonian telescope (catoptric) compared to a Schmidt cassegrainian telescope or the typical photo mirror lens (catadioptric), and finally, something like a Tessar or Plasmat are dioptrics.</p>

<p>The type lens you are dealing with is a Maksutov type with a Bouwers Camera variation. This type telescope uses a "thick meniscus" corrector plate (unlike the Schmidt corrector) which defocuses the light so that the spherical main mirror will then bring in a sharp focus to the secondary mirror. The thickmeniscus reduces the contrast at bit, often introduces some field curvature, and always causes some chromatic abberation. In the type lenses you are dealing with there is a simple lens system in the baffle tube (you can see it from the bottom) which corrects the chromatic abberation (this is the Bouwers Camera alteration). Even a truly fine mirror telephoto will sometimes have some lost contrast and field curvature although usually not of a serious nature (certainly no worse than most 35mm high speed lenses).</p>

<p>The last problem encountered in the long mirror lenses (or any lens longer than 200mm) is camera stability, use a tripod and wrap your hands around the lens tube and the camera itself. The tripod holds the camera in the right position and the soft tissue of your hands dampens the vibration introduced by mirror slap and the metal focal plane shutter.</p>

<p>Lynn</p>