How Equivalent is Equivalent?

[Inkanyezi]

Speed boosters have been available for telescopes for a long long time but called focal length reducers - aperture remains the same, focal length less so the F ratio is faster. Nothing magic about them as the image circle size is reduced increasing the concentration of light making it brighter. I reckon that they must reduce depth of field too. I'd guess if these don't reduce the image size on the sensor they must make use of light that doesn't currently hit it. Personally I would wait till rather a lot had been sold before buying one - vignetting for instance could be a problem if the whole image is needed. I suspect this patent will pend for ever. They are ever so expensive, see link to get a good idea of real cost.

http://agenaastro.com/antares-1-25-0...l-reducer.html

To be fair some are dearer.

Glad for the disclaimer. The speed booster or focal length reducer is in fact the opposite of a Barlow lens, instead of a negative system, it is positive.

In the case of focal length reducers for telescopes or Barlow lenses, mostly achromats of two elements glued together are used, and not much care is taken for optical errors beside chromatic aberration.

The speed booster for a camera however is a more complex system comprising a few more elements, just as the tele extenders could be of varying quality and varying price. some of them had few lenses, some had many more. I have a very good Vivitar extender with seven elements, as well as a four element Soligor extender, and the difference of image quality is apparent. (The tele extender is the photographic lens equivalent to a Barlow lens.)

The quality of the Metabones Speed Booster is declared in the whitepaper. Its production cost is comparable to a wide aperture, fast, photographic lens, but without the diaphragm and focusing mechanisms, and produced in rather low numbers, so it is no wonder that it also carries a price tag accordingly. It is not really the same as a small achromat to screw onto a telescope eyepiece. I regard it as pretty cheap really, considering the development costs and the small numbers and a very large variety of adaptations. I will certainly get one, to convert my Zeiss Planar 50 mm f/1.4 to a 35 mm f/1 portrait lens.

[Black Pearl]

I'd love to see a Zeiss 55mm f1.4 Otus used on say a Fuji X1-Pro or a NEX 7.

[ajohnw]

The quality of the Metabones Speed Booster is declared in the whitepaper. Its production cost is comparable to a wide aperture, fast, photographic lens, but without the diaphragm and focusing mechanisms, and produced in rather low numbers, so it is no wonder that it also carries a price tag accordingly. It is not really the same as a small achromat to screw onto a telescope eyepiece. I regard it as pretty cheap really, considering the development costs and the small numbers and a very large variety of adaptations. I will certainly get one, to convert my Zeiss Planar 50 mm f/1.4 to a 35 mm f/1 portrait lens.

These are fitted to truly diffraction limited apo telescopes with resolution figures that camera lenses can only dream of. The main reason for the cost is the fact that it's 4in diameter. Also the fact that it's Takahashi. They need to consider the actual characteristics of the glass that comes in - make some and even then check that they are ok.

http://www.optcorp.com/takahashi-toa130r-reducer.html

One of my own apo's is made by William Optics. Basically it will show very slight residual colour used visually at very high magnifications that are a lot higher than would be used for photographic work so some what cheaper. This is the same thing for that

http://www.williamoptics.com/wo_shop...product_id=649

Both makes of scope have mtf curves that could not be realistically achieved given the number of elements in photographic lenses other than by pure accident with a very low probability of ever occurring. Neither are made in very large numbers. Flattener? Gilding the lilly for the ultimate performance but often little difference in practice other than when very large sensors are used.

Of course as it's a white paper it must be correct.

John
-

[Black Pearl]

John, have you used a Speed Booster on a mirrorless camera?

[Sponge]

To change the subject a bit, I commonly see people recommending the use of a long focal length to "compress" the various elements in a distant landscape scene more than would happen using a short focal length. The corollary of your exercise is that the compression is the same whether using a long lens or cropping to the same field of view after using a short lens. This was proved to my satisfaction decades ago in a U.S. national magazine but most people still seem to think the degree of compression changes from focal length to focal length. Belief is so fervent that this is the first time I have mentioned it in over ten years.

I've always assumed that when someone mentions the compression from using a longer focal length on a distant scene they are referring (knowingly or not) to the compression from the far distance to subject in comparison to the same framing from a closer distance with a wider focal length (which looks very different and in many cases the same framing would be difficult to near impossible to achieve). I don't see much practicality in cropping a distant scene with a wide angle lens if there are better options.

[William W]

There are a few things mixed up in this discussion.

I concur.

Let's also not confuse some of what has been discussed thus far using the word "equivalent" (sometimes used quite loosely in meaning), to the Subject of "Equivalence" as it applies to Photography.

A good starting point for research on the topic of "Equivalence" is here: http://www.josephjamesphotography.co...ence/index.htm

WW

[William W]

I've always assumed that when someone mentions the compression from using a longer focal length on a distant scene they are referring (knowingly or not) to the compression from the far distance to subject in comparison to the same framing from a closer distance with a wider focal length (which looks very different and in many cases the same framing would be difficult to near impossible to achieve).

That is correct.

That is the meaning of: "compression" where its meaning is applicable to the visual impact of the final image.

That is how the term is used (technically) in text books and taught in Schools/Colleges/Universities.

WW

[William W]

Question:

Would that mean that if i use a 25mm on a m4/3 (x2) body, i would have the same view of a 50mm on a full frame, but would be unable to use that for portraits because of the distortions on the face (since 25mm is very wide)?

Response:

In a word, no.

I concur.

To answer differently: if you were happy with the portrait that you made with the a 50mm lens and 135 format camera (aka "Full Frame Camera) - then you will be just as happy with the SAME portrait made with your 4/3 Camera and a 25mm lens.

A simple explanation is:

You are discussing what is commonly (arguably wrongly) referred to as: “Perspective Distortion”.

Please note that one will be at exactly the same Shooting Distance to frame those two shots identically, in each of those two cameras.

“Perspective” is determined by ONLY two factors (the first being the major factor):
1. the DISTANCE from the Subject to the Film Plane (Sensor Plane)
2. the ELEVATION of the camera.
These two factors are often summarized technically as being “the Camera Viewpoint”.

Therefore to answer your question (assuming no major imperfections in either lens and that you are at a common portrait shooting distance), there will be absolutely NO difference in the “Perspective Distortion” seen in the result images of those two shots.

***

I know that 50mm lenses are ideal for portrait photography, but using a 50mm on a m4/3 would give me the equivalent of a 100mm view, which would have to place me quite far from my subject. Does that make sense?

Yes that makes sense.

But, I think that we need to define what a “portrait” is – a portrait is a photograph that is the likeness of a person or persons – although many folk think of a portrait as ONLY an Head and Shoulders Shot . . .

I also would encourage you to NOT to be handcuffed in your thinking that some particular lens is “ideal” for Portraiture – that is just silly thinking and will limit anyone’s creative potential.

Certainly seek to understand how different Focal Length Lenses will interpret different Portrait Shots: but no one lens is “ideal” of itself, for Portraiture.

I specifically mention this point because on many forums we will read something like:

"get a 50/1.8 it is really cheap and it is ideal Focal Length for portraiture and also it has shallow depth of field, and also perfect for low light work. Every camera bag should have a 50/1.8."

Comments like that, should be read in the context of the remark and the question being addressed - most usually a comment like that is a response to a question asked by a new photographer who has bought an APS-C format DSLR with the 18 to 55 kit lens and asks: "what is the next lens I should buy?"


Of course: a 50mm F/1.8 lens when used on an APS-C camera is a short telephoto lens and as such is very useful to use for many Portrait Shots.

WW

[Mike Buckley]

I've always assumed that when someone mentions the compression from using a longer focal length on a distant scene they are referring (knowingly or not) to the compression from the far distance to subject in comparison to the same framing from a closer distance with a wider focal length

I understand what you're explaining, which makes perfect sense, but I think the problem is that so many people simply don't understand the concept properly. I've heard people who I know didn't understand it assume that the long focal length mounted on a tripod is causing compression that would not occur if you left the tripod in place, changed to a shorter focal length, and cropped to achieve the same frame made with the longer focal length. They have actually bemoaned that if they didn't have the longer focal length they would not be able to achieve the same effect using a shorter focal length and cropping. That's probably why the magazine I mentioned published an article about it.

[William W]

. . . I think the problem is that so many people simply don't understand the concept properly. I've heard people who I know didn't understand it assume that the long focal length mounted on a tripod is causing compression that would not occur if you left the tripod in place, changed to a shorter focal length, and cropped to achieve the same frame made with the longer focal length. They have actually bemoaned that if they didn't have the longer focal length they would not be able to achieve the same effect using a shorter focal length and cropping. That's probably why the magazine I mentioned published an article about it.

ASIDE, btw I understood exactly that was the point you were making, my aim was to flush that explanation out whilst you were still online.

Thanks and cheers.

WW

[ajohnw]

No Robin I haven't. I have used them on telescopes though. A little different but not much. Some of the fit to anything types work well on slow say f15 to F10 telescopes and are probably really designed to suit them in the first place. The ones I linked to later are specifically designed for the telescopes they mention. I have also used the barlow's mentioned = converters on apo telescopes. The price of those at the higher end is similar to better makes of converter.

As you may have noted personally I would be a bit dubious and wait for a reasonably large number people to try them before even contemplating buying one. I would wonder about the wisdom of wanting F1 over F1.4 as well.

John
-

[ajohnw]

Might be worth mentioning that the crop for the same view is fine but when it's a portrait the face can't be cropped and features will be for shortened or extended.

There is an article on the wiki about this aspect. This is one of the shots. Notice that the front bottle is always the same size which takes magnification of the subject out of it.

http://en.wikipedia.org/wiki/File:Focal_length.jpg

It's a pity the camera seems to be off axis but that wouldn't change the result.

John
-

[ajohnw]

John, have you used a Speed Booster on a mirrorless camera?

I hadn't looked on the web for info about them and hadn't fully appreciated that they are intended to take full frame lenses on cropped sensors. That decreases the size of the image circle and boosts light levels. Makes more sense than trying to do the same with crop frame lenses.

Just did and see the sort of thing I expected to see - and still have doubts about the DOF aspect - converters increase it and boosters should do the opposite. The reason is the angle of the ray cones hitting the sensor. Simply put that is what gives rise to dof.

This is the review.

http://www.dpreview.com/articles/266...-speed-booster

John
-

[William W]

Might be worth mentioning that the crop for the same view is fine but when it's a portrait the face can't be cropped and features will be for shortened or extended.

I don't understand what you mean - I don't understand to what part of the many previous conversations you are referring.

WW

[ajohnw]

I don't understand what you mean - I don't understand to what part of the many previous conversations you are referring.

WW

The one you quoted. The distance to the subject doesn't matter. Compare one focal length to another and the perspective change will always be there. The only problem is that at some short distance the longer lens might only get part of the subject in. Say for instance at some distance only the nose of a person was captured. Basically if the eye view focal length range is around 100mm and was used the perspective of the nose would be correct. A 28mm lens would get the whole head in but the perspective would be incorrect. It all down to magnification which makes things look either further away or closer as far as a camera is concerned. The same applies to the features of what ever is being photographed. In other words a 100mm lens will give correct perspective of a tiny flower, some ones head, their entire body or even a scene.

Crop for the same view - the bottle shot link I posted shows that this wont work but the bottle looks the same in all - if you think about it as it has depth it can't be the same. If some one needs to take a flattering photo of some one with a rather large nose - long focal length maybe even up to 150mm. Opposite and if features would be "improved" - shorter focal length maybe even 50mm. Glamour model - debatable but I reckon 100mm.

John
-

[William W]

The distance to the subject doesn't matter. Compare one focal length to another and the perspective change will always be there.

Perspective is determined by Distance from Subject to Camera and the Camera Elevation.

Field of View is determined by the Focal Length of the Lens with respect to the Camera Format.

WW

[Sponge]

I understand what you're explaining, which makes perfect sense, but I think the problem is that so many people simply don't understand the concept properly. I've heard people who I know didn't understand it assume that the long focal length mounted on a tripod is causing compression that would not occur if you left the tripod in place, changed to a shorter focal length, and cropped to achieve the same frame made with the longer focal length. They have actually bemoaned that if they didn't have the longer focal length they would not be able to achieve the same effect using a shorter focal length and cropping. That's probably why the magazine I mentioned published an article about it.

Yes, I think even in a book or two that I have by professional photographers the term 'Lens Compression' is used which can be misleading. I just wanted to clarify that longer focal lengths have their place for distant landscape scenes.

[Inkanyezi]

I would wonder about the wisdom of wanting F1 over F1.4 as well.
-

The logic is simple. By using focal length reduction, the 50 mm lens behaves as it would have done on a 1.5x crop camera when mounted with the speed doubler on µ4/3. Thus the whole image circle is not used, as it would cover the crop format, which is larger. It is still a short tele lens, and those effects that are experienced toward the corners with Leica wide angle lenses on the NEX should not be apparent, particularly as the Speed Doubler makes the lens a bit more telecentric than without it. I expect it to give reasonable performance and the bokeh and background blur that I want at the large entrance pupil (36 mm) of the nominally 50 mm f/1.4 Planar (51 mm focal length). I can use it at closer distance to the subject when the focal length is shortened, than when it is equivalent to a 100 mm on FF. It should behave as a 70 mm f/2 would on FF, which I think is reasonable.

[ajohnw]

I had missed that aspect Urban until I read the review. Given that it's used on cropped sensors it makes more sense but I'll bet there will be penalties.

My comment was really based on F1. The bokeh builds up so rapidly even at F2 that it wont be easy to control it creatively. Not that many do that though.

Personally I would prefer an Olympus 45mm F1.8 that can do work like this



It's unusual to see a lens test shot like that and it isn't that an expensive lens even though it is designed for this sort of thing.

On bokeh build up I posted this recently which might interest you and shows just how rapidly it does built up on various crops and the apertures needed due to the crop factor. The reason that the 1.6x crop is slightly different is due to the available apertures. This unlike some comments account for the fact that dof circle of confusion size needs to be related to the sensor size when all are finally viewed at the same final size. They are just at some arbitrary distance - mug shot if I remember correctly. There must of course be some depth of field available for the subject. That is usually given as a max coc of 0.015mm for 12x8 images. 1/2 full frame hence the stop difference for equal blur. 0.015mm is about 0.09% sensor width. In most cases that will limit the fastest aperture that can be used. I chose the apertures on the basis of 300mm at 3m.
-

-
John
-

[loosecanon]

Just watched the Sky Arts programme on the Bailey exhibition in London. Interesting to see how many of the images were taken with "standard lens" eg 80mm on 2¼" . Many impact portraits show wide angle "distortion" which is perhaps why the make an impact.