Using achromats as wide angle correction lens

I've looked at all the different forum topics and this seems to be the place to put this question.

I've read a lot on wetpixel about the use of "close up" dioptres on wide angle lenses that are behind dome ports, as a sort of "lens correction lens" which incorporates the diopters close up ability allowing the lens to focus on the "apperant image" created by the dome port.

When I was researching which wide angle zoom lens to buy, I found that something called chromatic abberation (CA) could be expected with most wide angle zoom lenses, due to the different colour wavelengths hitting the lens optics at different angles (or something like that), showing up on the picture as a slight colour blur at the edges of the shot, especially in the corners.

Then I started reading about close up lenses that screw onto the front of lenses, and found that there are two distinct types, single element, and ACHROMATS ( two glass elements) the second type being specifically designed to reduce or eliminate CA as the light passed through the dioptre before hitting the lens optics, so I got an ACROMAT close up, primarily to increase the close focusing ability of the macro lenses I have.

But I started thinking, could it double as a correction lens for my 12/24 when I put it behind my dome port? It wont improve any CA that the main lens produces, but it wont add to, or magnify any CA, as a single element dioptre might.

So I fitted the ACHROMAT to my wide zoom and, as expected, the vignetting around the shot at 12mm was almost circular! This, I realised is due to the depth of the mounting ring of the dioptre, which also acts as a protector for the glass. I thought about it a bit more and there is a company that may be able to remount the ACHROMAT glass into a new, thinner ring therefor overcoming the vignetting problem, with luck they shoud be able to keep the original mounting ring intact so I can screw this back on to the ACHROMAT to protect it when it isn't being used for wide work.

SO! Eventually, here is my question. These ACHROMAT lenses are usually fitted to "long" lenses,

( The CANON 500D (+2) is recommended for lenses between 70mm and 300mm.)

where the image or light input, even at the edges of the shot, is coming virtually straight down the barrel of the lens. Where as the light input from a 12/24mm lens, at its widest is coming to the lens at a much more acute angle, ( even taking into account that on a DX sensor the light angle is closer to 18mm.) Befor I go to the expense of having this piece of glass remounted do you think this will cause problems, with the edges of my shot not being able to focus through the dioptre?

I've looked at all the different forum topics and this seems to be the place to put this question.

 

I've read a lot on wetpixel about the use of "close up" dioptres on wide angle lenses that are behind dome ports, as a sort of "lens correction lens" which incorporates the diopters close up ability allowing the lens to focus on the "apperant image" created by the dome port.

 

When I was researching which wide angle zoom lens to buy, I found that something called chromatic abberation (CA) could be expected with most wide angle zoom lenses, due to the different colour wavelengths hitting the lens optics at different angles (or something like that), showing up on the picture as a slight colour blur at the edges of the shot, especially in the corners.

 

Then I started reading about close up lenses that screw onto the front of lenses, and found that there are two distinct types, single element, and ACHROMATS ( two glass elements) the second type being specifically designed to reduce or eliminate CA as the light passed through the dioptre before hitting the lens optics, so I got an ACROMAT close up, primarily to increase the close focusing ability of the macro lenses I have.

But I started thinking, could it double as a correction lens for my 12/24 when I put it behind my dome port? It wont improve any CA that the main lens produces, but it wont add to, or magnify any CA, as a single element dioptre might.

 

So I fitted the ACHROMAT to my wide zoom and, as expected, the vignetting around the shot at 12mm was almost circular! This, I realised is due to the depth of the mounting ring of the dioptre, which also acts as a protector for the glass. I thought about it a bit more and there is a company that may be able to remount the ACHROMAT glass into a new, thinner ring therefor overcoming the vignetting problem, with luck they shoud be able to keep the original mounting ring intact so I can screw this back on to the ACHROMAT to protect it when it isn't being used for wide work.

 

SO! Eventually, here is my question. These ACHROMAT lenses are usually fitted to "long" lenses,

 

( The CANON 500D (+2) is recommended for lenses between 70mm and 300mm.)

 

where the image or light input, even at the edges of the shot, is coming virtually straight down the barrel of the lens. Where as the light input from a 12/24mm lens, at its widest is coming to the lens at a much more acute angle, ( even taking into account that on a DX sensor the light angle is closer to 18mm.) Befor I go to the expense of having this piece of glass remounted do you think this will cause problems, with the edges of my shot not being able to focus through the dioptre?

I messed that up!

The 12-24mm is pretty unhappy behind a dome port, but in my experience the problem is with edge softness and not, particularly, chromatic aberration.

Achromats are thick, and that must infringe onto the edges of the image: I don't think that you can substantially reduce this with a new mounting ring.

The bigger the dome, the less the optical disturbance, if the dome is at the proper distance from the lens.

The Nexus 170 dome is better than the custom dome that Nexus make, in some respects, but the custom dome is smaller, and glass. Subal's dome seems to be OK, and the larger domes from Ikelite and Sea & Sea are OK, too, I believe.

I have been using the smaller, custom glass dome from Nexus, and I'm not even sure about the diopter: I use a plain URPro filter sometimes, and sometimes a +2D with strobes: can you see much difference in the edge distortion?

No diopter (Magic Filter):

+2D (strobes)

Tim

B)

Without going into the theory behind the use of diopters and whether achromats are better than simple diopters, I will say that I have tried both and that as a very generalised 'rule of thumb', smaller domes will require a +3 or even +4 whilst larger domes will require a +2, and in my experience there is little to be gained by using an achromat. Whilst I have found them to produce slightly better central definition, the corners may even be marginally worse when using an achromat. Also if a thick achromat is used then placement of the dome may need to be adjusted too. My own recommendation would be to use a simple diopter from a reputable manufacturer - currently I'm using a Marumi +3 behind a smallish port and find its probably producing as good results as I can expect to get from my port/lens combination.

Thank you both for your prompt replies, looks like I will keep the achromat intact for extreme macro. I'm using a Sea and Sea housing with a compact dome port which is a 6 inch diameter port and not a full hemisphere dome. I try not to have the front of the lens going too far into the port, ideally keeping it on the same plane as the back plate of the port putting as much air between the lens and the water as possible, but I still find the use of dioptres helpful. Thanks again, I think you may have saved me a bit of money

Stefin,

The position of the lens in the dome depends solely upon the location of the nodal point inside the lens. You want to make sure you have the lens positioned so that the center of curvature of the dome hits that nodal point.

Cheers

James